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UNITED STATES
SECURITIES AND EXCHANGE COMMISSION
Washington, D.C. 20549

FORM 10-K

Aon Corporation
(Exact Name of Registrant as Specified in Its Charter)

        Indicate by check mark whether the registrant: (1) has filed all reports required to be filed by Section 13 or 15(d) of the Securities Exchange Act of 1934 during the preceding 12 months (or for such shorter period that the registrant was required to file such reports), and (2) has been subject to such filing requirements for the past 90 days. YES ý    NO o

        Indicate by check mark if disclosure of delinquent filers pursuant to Item 405 of Regulation S-K is not contained herein, and will not be contained, to the best of registrant's knowledge, in definitive proxy or information statements incorporated by reference in Part III of this Form 10-K or any amendment to this Form 10-K.    o

        Indicate by check mark whether the registrant is an accelerated filer (as defined in Exchange Act Rule 12b-2). YES ý    NO o

        Aggregate market value of common stock held by non-affiliates of the registrant as of June 30, 2004 was $8,297,300,700.

        Number of shares of common stock outstanding as of February 28, 2005 was 317,464,955.

Documents incorporated by reference:

        Portions of Aon Corporation's Proxy Statement for the 2005 Annual Meeting of Stockholders to be held on May 20, 2005 are incorporated by reference in this Form 10-K in response to Part III, Items 10, 11, 12, 13 and 14.

PART I

Item 1.    Business

Overview

        Aon Corporation ("Aon"), through its various subsidiaries worldwide, serves its clients through three operating segments:

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Risk and Insurance Brokerage Services acts as an advisor and insurance broker, helping clients manage their risks, and negotiating and placing insurance risk with insurance carriers through our global distribution network.



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Consulting provides advice and services to clients for employee benefits, compensation, management consulting, communications, human resource outsourcing and strategic human resource consulting.



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Insurance Underwriting provides specialty insurance products, including supplemental accident, health and life insurance; credit life, accident and health insurance; extended warranty products, and select property and casualty insurance products and services.

        Our clients include corporations and businesses, insurance companies, professional organizations, independent agents and brokers, governments, and other entities. We also serve individuals through personal lines, affinity groups, and certain specialty operations.

        Incorporated in 1979, Aon is the parent corporation of long-established and more recently formed companies. Aon has approximately 48,000 employees and does business in more than 120 countries and sovereignties.

Segment Operations

Risk and Insurance Brokerage Services

        The Risk and Insurance Brokerage Services segment generated approximately 57% of our total operating segment revenues in 2004. This is the largest of our operating segments, with approximately 31,000 employees worldwide. Risk and insurance brokerage services are provided by certain indirect subsidiaries, including Aon Risk Services Companies, Inc.; Aon Holdings International bv; Aon Re Worldwide, Inc.; Aon Limited (U.K.); and Cananwill, Inc.

Subsegments

        We measure our revenues in this segment under the following areas:

        Risk Management and Insurance Brokerage encompasses our retail and wholesale brokerage services, affinity products, managing general underwriting, placement and captive management services, and premium finance services for small, mid-sized and large companies, including Fortune 500 corporations. The Americas' operations provide products and services to clients in North and South America, the Caribbean and Bermuda. Our International operations offer similar products and services to the rest of the world. Risk management services also include risk identification and assessment, safety engineering, claims and loss cost management, and program administration.

        Retail brokerage has practice areas to deliver specialized advice and services in such segments as entertainment, media, financial institutions, marine, aviation, construction, healthcare and energy, among others.

        As a retail broker, Aon generally serves as an advisor to corporate clients and can arrange a wide spectrum of risk management solutions, including property, general liability, professional and directors' and officers' liability, workers' compensation, and other exposures. Aon also provides affinity products

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for professional liability, life, disability income and personal lines for individuals, associations and businesses.

        Wholesale brokerage operations serve retail insurance brokers and independent agents in placing large and small accounts with both standard and specialty carriers. Our wholesale brokerage operations are a commercial resource for insurance products, specialty programs and exclusive underwriting facilities.

        In our managing general underwriting business, we provide outsourced solutions to insurance companies, such as risk selection, premium rating, form design and client service.

        Aon's wholesale brokers and managing general underwriting units offer more than 450 insurance products and programs. Clients may access them directly, or through the Aon Specialty Product Network (ASPN), which we developed as a single-point-of-contact for agent and broker clients who need specialty insurance solutions for their customers.

        We are also a major provider in the management of captive insurance companies that enable our clients to manage risks that would be cost prohibitive or unavailable in traditional insurance markets.

        Reinsurance Brokerage and Related Services offers sophisticated advisory services in program design and claim recoveries that enhance the risk/return characteristics of insurance policy portfolios, improve capital utilization and evaluate and mitigate catastrophic loss exposures worldwide. An insurance or reinsurance company may seek reinsurance or other risk-transfer financing on all or a portion of the risks it insures. Brokerage services use dynamic financial analysis and capital market alternatives, such as transferring catastrophe risk through securitization.

        Aon Re Worldwide, Inc., its subsidiaries and its affiliates provide reinsurance services to insurance and reinsurance companies and other risk assumption entities by acting as brokers or intermediaries on all classes of reinsurance. While property and casualty lines dominate, our reinsurance activities also include specialty lines such as professional liability, medical malpractice, accident, life and health. Services include advice, placement of reinsurance and alternative risk transfer financing with capital markets, and related services such as actuarial, financial and regulatory consulting, portfolio analysis, catastrophe modeling, and claims services.

        Claim Services offered claims administration and loss cost management services through dedicated subsidiaries that are separate from our risk management and reinsurance brokerage services. In the United States, these services were delivered principally through Cambridge Integrated Services Group ("Cambridge"). During 2004, we exited most of these activities by completing the sale of our U.K. claims operations in the second quarter 2004 and our Cambridge business in fourth quarter 2004.

Compensation for Services

        Revenues are generated through commissions, fees from clients, and compensation from insurance and reinsurance companies with whom we place business for services provided to them. On October 22, 2004, we announced that we are terminating our contingent commission arrangements with underwriters. We have nearly completed this process and we are working closely with our clients, insurance carriers, regulators and others to establish a new business model that is transparent, easy to understand and accepted by clients.

        Commission rates and fees vary, depending upon several factors which may include the amount of premium, the type of insurance or reinsurance coverage provided, the particular services provided to an insurer or reinsurer, and the capacity in which the broker acts. We also receive investment income on funds held on behalf of clients.

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Competitive Conditions

        We believe we are the second largest insurance broker worldwide based on total revenues. The risk and insurance brokerage services business is highly competitive and we compete with two other global brokers in addition to numerous specialist, regional and local firms in almost every area of our business; insurance and reinsurance companies that market and service their insurance products without the assistance of brokers or agents; and with other businesses, including commercial and investment banks, accounting firms and consultants that provide risk-related services and products.

Consulting

        The Consulting segment generated approximately 12% of our total operating segment revenues in 2004. It has approximately 7,000 employees worldwide located in more than 120 offices, with operations in the United States, Canada, Europe, Asia/Pacific and South Africa. We believe we are the world's third largest employee benefit consultant and the second largest in the United States based on total revenues.

Subsegments

        Through our Aon Consulting Worldwide, Inc. subsidiary (Aon Consulting), we provide a full range of human capital consulting services in two subsegments (Consulting Services and Outsourcing) that operate in six practice areas:

        Employee Benefits advises clients regarding the structure, funding and administration of employee benefit programs, which attract, retain and motivate employees. Benefits consulting includes health and welfare, retirement, executive benefits, absence management, compliance, employee commitment, investment advisory and elective benefits services.

        Human Resource Outsourcing offers employment processing, performance improvement, benefits administration, and other employment-related services.

        Compensation focuses on designing salary, bonus, commission, stock option, and other pay structures, with special expertise in the financial services and technology industries.

        Management Consulting assists clients in process improvement and design; leadership, organization and human capital development; and change management.

        Communications advises clients on how to communicate initiatives that support their corporate vision.

        Strategic Human Resource Consulting advises complex global organizations on talent, change and organization effectiveness issues including assessment, selection performance management, succession planning, organization design and related people-management programs.

        Aon Consulting works to maximize the value of clients' human resources spending, increase employee productivity, and improve employee performance. Its approach addresses a trend toward more diverse workforces (demographics, nationalities, cultures and work/lifestyle preferences) that require more choices and flexibility among employers—with benefit options suited to individual needs.

        Our consulting professionals and their clients also identify options in human resource outsourcing and process improvement. Prime areas where companies choose to use outsourcing services include the assessment and selection of job candidates, employment processing, training and development, benefits administration and the individual benefits enrollment process.

Compensation for Services

        Aon Consulting revenues are principally derived from fees paid by clients for advice and services. In addition, commission revenue is received from insurance companies for the placement of individual

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and group insurance contracts, primarily life, health and accident coverages. On October 22, 2004, we announced that we are terminating our contingent commission arrangements with underwriters. We have nearly completed this process and we are working to establish a new business model that is transparent, easy to understand, and accepted by clients.

Competitive Conditions

        Our consulting business faces strong competition from other privately and publicly held worldwide and national consulting companies, as well as regional and local firms. Competitors include independent consulting firms and consulting organizations affiliated with accounting, information systems, technology and financial services firms. Some of our competitors provide administrative or consulting services as an adjunct to other primary services.

Insurance Underwriting

        Our insurance underwriting segment, with approximately 9,000 employees worldwide, has operations in the United States, Canada, Latin America, Europe and Asia/Pacific. This segment generated approximately 31% of Aon's total operating segment revenues in 2004.

Subsegments

        We classify our insurance underwriting businesses into two sub-segments: (1) accident & health and life, and (2) warranty, credit and property and casualty.

Accident & Health and Life

        Our Combined Insurance Company of America and Combined Life Insurance Company of New York (Combined) subsidiaries provide supplemental accident, health and life insurance. We are a leading underwriter and distributor of specialty individual accident, disability, health and life insurance products that are targeted to middle income consumers in the United States, Europe, Canada, and Asia/Pacific.

        A worldwide sales force of approximately 6,800 exclusive career agents service clients regularly to initiate and renew coverage and to sell additional coverage. We offer a wide range of accident and sickness insurance products, including short-term disability, critical conditions and cancer aid, Medicare products, hospital confinement/recovery, and long-term care coverage. Most of these products are primarily fixed-indemnity obligations, and are not subject to escalating medical cost inflation.

        With the continuing rise of employee benefit costs, Combined also works with employers to contain those costs while offering quality benefits that appeal to their employees' individual needs. For example, a Worksite Solutions program complements existing benefits packages offered by employers with no additional cost to a company. Individual employees choose among supplemental insurance product options and pay for them through payroll deductions.

Compensation for Services

        Accident and health revenues are based on premiums paid by policyholders for insurance coverage and services.

Competitive Conditions

        The accident and health insurance industry in the United States is highly diverse, with more than 1,500 accident and health and life insurance companies competing in various segments of the industry. We believe that competition in our accident, health and life business is based on service, product features, price, commission structure, financial strength, claims-paying ability ratings and name recognition.

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Warranty, Credit and Property and Casualty

        We believe we are the world's largest independent provider of extended warranty products. These products are offered through our Virginia Surety Company, Inc. and London General Insurance Company Limited subsidiaries.

        Extended warranty, which is the largest line of business in this sub-segment, offers extended service plans and warranties for:

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some of the world's premier manufacturers, distributors and retailers of many types of consumer goods including automobiles, electronics, appliances, computers and telephone equipment



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home buyers and sellers.

        Other products include extended warranty or insurance protection for items purchased with a credit card and extended warranties on major home systems and appliances.

        Services include compliance support, merchandising, direct marketing, training, and customer care management services. Products are sold through retailers, automotive dealers, insurance agents and brokers, and real estate brokers.

        Our credit life, accident and health, and disability insurance provides coverage for unpaid loans in the event of death, illness, accident or involuntary unemployment. This insurance is sold by automobile dealers with automobile financing, and by financial institutions with consumer loans.

        Select property and casualty products are designed to protect businesses against losses related to various personal and commercial risks, such as professional liability errors and omissions, excess liability, and workers' compensation. We offer select commercial property and casualty business on a limited basis through managing general underwriters, primarily Aon-owned companies.

Compensation for Services

        Insurance revenues are based on premiums paid by policyholders. Certain other revenues are based on fees paid by clients for administrative and other services.

Competitive Conditions

        We believe that competition in our warranty, credit and specialty property and casualty business is based on service, product features, price, commission structure, financial strength, claims-paying ability, ratings and name recognition. In our extended warranty business, we compete with a large number of insurance companies and other financial services providers in addition to third-party administrators, manufacturers and distributors.

Disposal of Operations

        The Registrant hereby incorporates by reference Note 5, "Disposal of Operations," of the Notes to Consolidated Financial Statements in Part II, Item 8 of this report.

Licensing and Regulation

        Regulatory authorities in the states or countries in which the operating subsidiaries of Aon Group conduct business may require individual or company licensing to act as brokers, agents, third party administrators, managing general agents, reinsurance intermediaries or adjusters. Under the laws of most states in the U.S. and of most foreign countries, regulatory authorities have relatively broad discretion with respect to granting, renewing and revoking brokers' and agents' licenses to transact business in the state or country. The manner of operating in particular states and countries may vary according to the licensing requirements of the particular state or country, which may require, among other things, that a firm operate in the state or country through a local corporation. In a few states and

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countries, licenses are issued only to individual residents or locally owned business entities. In such cases, Aon Group subsidiaries have arrangements with residents or business entities licensed to act in the state or country.

        Insurance companies must comply with laws and regulations of the jurisdictions in which they do business. These laws and regulations are designed to ensure financial solvency of insurance companies and to require fair and adequate service and treatment for policyholders. They are enforced by the states in the U.S., by the Financial Services Authority ("FSA") in the United Kingdom, and by various regulatory agencies in other countries through the granting and revoking of licenses to do business, licensing of agents, monitoring of trade practices, policy form approval, minimum loss ratio requirements, limits on premium and commission rates, and minimum reserve and capital requirements. Compliance is monitored by the state insurance departments through periodic regulatory reporting procedures and periodic examinations. The quarterly and annual financial reports to the regulators in the U.S. utilize statutory accounting principles which are different from U.S. generally accepted accounting principles. The statutory accounting principles, in keeping with the intent to assure the protection of policyholders are based, in general, on a liquidation concept, while U.S. generally accepted accounting principles are based on a going-concern concept.

        The state insurance regulators are members of the National Association of Insurance Commissioners ("NAIC"). The NAIC seeks to promote uniformity of, and to enhance the state regulation of, insurance. Both the NAIC and the individual states continue to focus on the solvency of insurance companies and their conduct in the marketplace. This focus is reflected in additional regulatory oversight by the states and emphasis on the enactment or adoption of a series of NAIC model laws and regulations designed to promote solvency.

        The NAIC has a formula for analyzing insurers called risk-based capital ("RBC"). RBC establishes "minimum" capital threshold levels that vary with the size and mix of a company's business. It is designed to identify companies with capital levels that may require regulatory attention.

        The state insurance holding company laws require prior notice to, and approval of, the domestic state insurance department of intracorporate transfers of assets within the holding company structure, including the payment of dividends by insurance company subsidiaries. In addition, the premium finance loans by Cananwill, our indirect wholly owned subsidiary, are subject to one or more truth-in-lending and credit regulations, insurance premium finance acts, retail installment sales acts and other similar consumer protection legislation. Failure to comply with such laws or regulations can result in the temporary suspension or permanent loss of the right to engage in business in a particular jurisdiction as well as other penalties.

        Beginning in January 2005, our principal subsidiary in the U.K., Aon Limited, must be authorized by the FSA. Previously, Aon Limited was a member of a self-regulatory body. Regulation by the FSA has been introduced pursuant to the European Insurance Mediation Directive, which sets minimum standards for those involved in advising on, arranging, administering or introducing contracts of insurance. The regulation requires significant operational changes, for example, enhanced disclosures, particularly in connection with retail (private and non-commercial) customers. The FSA has also indicated that it will adopt rules regarding use of funds held on behalf of clients that will have significant consequences for all brokers operating in the London market.

Clientele

        No significant part of our or our subsidiaries' business is dependent upon a single client or on a few clients, the loss of any one of which would have a material adverse effect on us or our operating segments.

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Employees

        At December 31, 2004, our operating subsidiaries had approximately 48,000 employees, of whom approximately 44,500 are salaried and hourly employees and the remaining 3,500 are career agents who are generally compensated wholly or primarily by commission. In addition, there were approximately 3,300 international career agents who are considered independent contractors and are not our employees. Of the total number of employees, approximately 21,000 work in the U.S.

Risks Related to Our Business and the Insurance Industry

Our results may fluctuate due to many factors, including cyclical or permanent changes in the insurance and reinsurance industries.

        Our results historically have been subject to significant fluctuations arising from uncertainties and changes in the insurance industry. Changes in premium rates affect not only the potential profitability of our underwriting businesses but also generally affect the commissions and fees payable to our brokerage businesses. In addition, insurance industry developments that can significantly affect our financial performance include factors such as:

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rising levels of actual costs that are not known by companies at the time they price their products;



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volatile and unpredictable developments, including weather-related and other natural and man-made catastrophes, including acts of terrorism;



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changes in levels of capacity and demand, including reinsurance capacity;



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changes in reserves resulting from the general claims and legal environments as different types of claims arise and judicial interpretations relating to the scope of insurers' liabilities develop; and



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changes in business practices and business compensation models.

Our results may be adversely affected if we are unable to successfully implement a new business model.

        In October 2004, we announced that we were terminating contingent commission arrangements with underwriters. In connection with the elimination of contingent commissions, we are in the process of establishing a new business compensation model. There is no assurance that we will be able to develop an effective new business compensation model, nor can we assure that any new business compensation model we develop will generate revenues equivalent to those previously received from contingent commissions.

A further decline in the credit ratings of our senior debt and commercial paper may adversely affect our borrowing costs and financial flexibility.

        In 2004, Standard & Poor's (S&P) lowered its ratings on our senior debt to the current rating of "BBB+" from "A-". In addition, S&P placed all their ratings for Aon on credit watch with negative implications. Also in 2004, Moody's Investor Services (Moody's) and Fitch, Inc. placed both our senior debt and commercial paper ratings on negative outlook and credit watch with negative implications, respectively.

        In March 2005, Fitch, Inc. lowered its ratings on our senior debt from "A-" to "BBB+" and affirmed our commercial paper rating of "F2." Their rating outlook continues to be negative. S&P affirmed its ratings for Aon and removed us from credit watch. Moody's affirmed its ratings on our senior debt and changed their outlook from negative to stable. A downgrade in the credit ratings of our senior debt and commercial paper would increase our borrowing costs and reduce our financial flexibility.

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        Any such further downgrade may trigger obligations of our company to fund certain amounts with respect to our premium finance securitizations, including, in the event of a downgrade by Moody's prior to April 28, 2005, up to $61 million with respect to our U.S. facility. Moreover, some of our debt instruments, such as our 6.20% notes due January 2007 ($250 million of which are outstanding), expressly provide for interest rate increases in the case of certain ratings downgrades. Similarly, any such downgrade would increase our commercial paper interest rates or may result in our inability to access the commercial paper market altogether. We cannot assume that our financial position would not be adversely affected if we are unable to access the commercial paper market. A downgrade in the credit ratings of our senior debt may also adversely affect the claims-paying ability or financial strength ratings of our insurance company subsidiaries. See "A decline in the financial strength or claims-paying ability ratings of our insurance underwriting subsidiaries may increase policy cancellations and negatively impact new sales of insurance products" below.

We face significant competitive pressures in each of our businesses.

        We believe that competition in our lines of business is based on service, product features, price, commission structure, financial strength, claims-paying ability ratings and name recognition. In particular, we compete with a large number of national, regional and local insurance companies and other financial services providers, brokers, and with respect to our extended warranty business, third-party administrators, manufacturers and distributors.

        Some of our underwriting competitors have penetrated more markets and offer a more extensive portfolio of products and services and have more competitive pricing than we do, which can adversely affect our ability to compete for business. Some underwriters also have higher claims-paying ability ratings and greater financial resources with which to compete and are subject to less government regulation than our underwriting operations.

        We encounter strong competition for both business and professional talent in our insurance brokerage and risk management services operations from other insurance brokerage firms which also operate on a nationwide or worldwide basis, from a large number of regional and local firms throughout the world, from insurance and reinsurance companies that market and service their insurance products without the assistance of brokers or agents, and from other businesses, including commercial and investment banks, accounting firms and consultants that provide risk-related services and products. Our consulting operations compete with independent consulting firms and consulting organizations affiliated with accounting, information systems, technology and financial services firms around the world.

        In addition, the increase in competition due to new legislative or industry developments could adversely affect us. These developments include:

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an increase in capital-raising by insurance underwriting companies, which could result in new entrants to our markets and an influx of capital into the industry;



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the selling of insurance by insurance companies directly to insureds;



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changes in our business compensation model as a result of regulatory investigations;



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the establishment of programs in which state-sponsored entities provide property insurance in catastrophe prone areas or other alternative markets types of coverage; and



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additional regulations promulgated by the Financial Services Authority in the U.K.

        New competition as a result of these developments could cause the supply of, and demand for, our products and services to change, which could adversely affect our results of operations and financial condition.

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A decline in the financial strength or claims-paying ability ratings of our insurance underwriting subsidiaries may increase policy cancellations and negatively impact new sales of insurance products.

        Financial strength and claims-paying ability ratings have become increasingly important factors in establishing the competitive position of insurance companies. These ratings are based upon criteria established by the rating agencies for the purpose of rendering an opinion as to an insurance company's financial strength, operating performance, strategic position and ability to meet its obligations to policyholders. They are not evaluations directed toward the protection of investors, nor are they recommendations to buy, sell or hold specific securities. Periodically, the rating agencies evaluate our insurance underwriting subsidiaries to confirm that they continue to meet the criteria of the ratings previously assigned to them. A downgrade, or the potential for a downgrade, of these ratings could, among other things, increase the number of policy cancellations, adversely affect relationships with brokers, retailers and other distributors of our products and services, negatively impact new sales and adversely affect our ability to compete.

        Virginia Surety Company, Inc., our principal property and casualty insurance company subsidiary, is currently rated "A-" (excellent; fourth highest of 16 rating levels) by A.M. Best Company. Combined Insurance Company of America, the principal insurance subsidiary that underwrites our specialty accident and health insurance business, is currently rated "A" (excellent; third highest of 16 rating levels) by A.M. Best Company, "BBB+" (good; fourth highest of nine rating levels and highest ranking within the level) for financial strength by S&P and "A3" (good; third highest of nine rating levels) for financial strength by Moody's Investors Service. We cannot assure that one or more of the rating agencies will not downgrade or withdraw their financial strength or claims-paying ability ratings in the future.

Changes in interest rates and investment prices could reduce the value of our investment portfolio and adversely affect our financial condition or results.

        Our insurance underwriting subsidiaries own a substantial investment portfolio of fixed-maturity and equity and other long-term investments. As of December 31, 2004, our fixed-maturity investments (approximately 97% was investment grade) had a carrying value of $3.5 billion, our equity investments had a carrying value of $40 million and our other long-term investments and limited partnerships had a carrying value of $483 million. Accordingly, changes in interest rates and investment prices could reduce the value of our investment portfolio and adversely affect our financial condition or results.

        For example, changes in domestic and international interest rates directly affect our income from, and the market value of, fixed-maturity investments. Similarly, general economic conditions, stock market conditions and other factors beyond our control affect the value of our equity investments. We monitor our portfolio for other-than-temporary impairments in carrying value. For securities judged to have an other-than-temporary impairment, we recognize a realized loss through the statement of income to write down the value of those securities.

        For 2004, we recognized impairment losses of $3 million. We cannot assure that we will not have to recognize additional impairment losses in the future, which would negatively affect our financial results.

        On December 31, 2001, our two major insurance companies sold the vast majority of their limited partnership portfolio, valued at $450 million, to Private Equity Partnership Structures I, LLC, (PEPS I) a qualifying special purpose entity (QSPE). The common stock interest in PEPS I is held by a limited liability company which is owned by one of our subsidiaries (49%) and by a charitable trust, which is not controlled by us, established for victims of the September 11 attacks (51%). Approximately $171 million of investment grade fixed-maturity securities were sold by PEPS I to unaffiliated third parties. PEPS I then paid our insurance underwriting companies the $171 million in cash and issued to them an additional $279 million in fixed-maturity and preferred stock securities. The fixed-maturity

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securities our insurance underwriting companies received from PEPS I are rated as investment grade by S&P.

        As part of this transaction, our insurance underwriting companies were required to purchase from PEPS I additional fixed-maturity securities in an amount equal to the unfunded limited partnership commitments, as they are requested. Beginning in July 2004, Aon Parent Company is funding all future commitments. As of December 31, 2004, these unfunded commitments amounted to $60 million.

        Although the PEPS I transaction has reduced the reported earnings volatility historically associated with directly owning limited partnership investments, it will not eliminate our risk of future losses. For instance, we must analyze our preferred stock and fixed-maturity interests in PEPS I for other-than-temporary impairment, based on the valuation of the limited partnership interests held by PEPS I, and recognize an impairment loss if necessary. We cannot assure that we will not have to recognize impairment losses with respect to our PEPS I interests in the future.

        The FASB has a current project on its agenda that is expected to result in a change to U.S. generally accepted accounting principles with respect to financial asset transfers such as the PEPS I transaction. We cannot assure that the current accounting for our PEPS I investments will be unaffected by these possible changes.

Our net pension liabilities may continue to grow, which could adversely affect our stockholders' equity, net income, cash flow and liquidity, and require us to make additional cash contributions to the pension plans.

        To the extent that the present value of the benefits incurred to date for pension obligations in the major countries in which we operate continue to exceed the market value of the assets supporting these obligations, our financial position and results of operations may be adversely affected. Primarily as a result of the decline in the equity markets over the past several years, some of our defined benefit pension plans, particularly in the U.K., have suffered significant valuation losses in the assets backing the related pension obligation.

        Current projections indicate that our 2005 defined benefit pension expense for our major pension plans would increase by approximately $18 million compared with 2004 and that cash contributions of approximately $202 million would be required in 2005. Total cash contributions to these major defined benefit pension plans in 2004 were $189 million, a decrease of $28 million over 2003. Future estimates are based on certain assumptions, including discount rates, interest rates, fair value of assets for some of our plans and expected return on plan assets. We are currently taking actions to manage our pension liabilities, including closing certain plans to new participants. However, changes in our pension benefit obligations and the related net periodic costs or credits may occur in the future due to any variance of actual results from our assumptions and changes in the number of participating employees. As a result, there can be no assurance that we will not experience future decreases in stockholders' equity, net income, cash flow and liquidity or that we will not be required to make additional cash contributions in the future beyond those which have been announced.

We are subject to a number of contingencies and legal proceedings which, if determined unfavorably to us, would adversely affect our financial results.

        We are subject to numerous claims, lawsuits, regulatory matters and tax assessments in connection with our businesses. The damages that may be claimed are substantial, including, in many instances, claims for punitive or extraordinary damages. The litigation naming us as a defendant ordinarily involves our activities as a broker or provider of insurance products or as an employer. It is possible that, if the outcomes of these contingencies and legal proceedings were not favorable to us, it could materially adversely affect our future financial results. In addition, our results of operations, financial condition or liquidity may be adversely affected if in the future our insurance coverage proves to be inadequate or unavailable or there is an increase in liabilities for which we self-insure.

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        On April 21, 2004, Aon received a subpoena from the Office of the Attorney General of the State of New York calling for the production of documents relating to Placement Service Agreements, Market Service Agreements and similar agreements under which insurance carriers pay compensation to Aon beyond standard commissions. The office subsequently issued several other requests for information to Aon as part of its inquiry into alleged practices in the insurance industry, including bid-rigging, fictitious quotes, "tying," and "steering" of business. The departments of insurance or attorneys general of approximately 25 other states have also issued subpoenas or requested information regarding these and other issues. Aon is fully cooperating with all of these investigations.

        Purported clients have also filed civil litigation against Aon and other companies under a variety of laws and legal theories relating to broker compensation practices and other issues under investigation by New York and other states. As previously reported, a putative class action styled Daniel v. Aon (Affinity) has been pending in the Circuit Court of Cook County, Illinois since August 1999. On July 28, 2004, the Court granted plaintiff's motion for class certification. On March 9, 2005, the Court gave preliminary approval to a nationwide class action settlement within the $40 million reserve established in the fourth quarter of 2004.

        Beginning in June 2004, a number of other putative class actions have been filed against Aon and other companies by purported clients under a variety of legal theories, including state tort, contract, fiduciary duty, and statutory theories, and federal antitrust and the Racketeer Influenced and Corrupt Organizations Act theories. These actions are currently pending at early stages in state court in California and Florida and in federal court in Illinois, South Carolina and New Jersey. Aon believes it has meritorious defenses in all of these cases, and intends to vigorously defend itself against these claims. The outcomes of these lawsuits, and any losses or other payments that may occur as a result, cannot be predicted at this time.

        Beginning in late October and early November 2004, several putative securities class actions have been filed against Aon in the United States District Court for the Northern District of Illinois. Also beginning in late October and early November 2004, several putative ERISA class actions were filed against Aon in the United States District Court for the Northern District of Illinois. Aon believes it has meritorious defenses in all of these cases, and intends to vigorously defend itself against these claims. The outcomes of these lawsuits, and any losses or other payments that may occur as a result, cannot be predicted at this time.

        In early February 2005 the Company received a subpoena from the U.S. Department of Labor regarding compensation arrangements in connection with clients' employee benefit plans. The Company is cooperating with the investigation.

        Although the ultimate outcome of all matters referred to above cannot be ascertained, and liabilities in indeterminate amounts may be imposed on us, on the basis of present information, amounts already provided, availability of insurance coverages and legal advice received, it is the opinion of management that the disposition or ultimate determination of such claims will not have a material adverse effect on the consolidated financial position of Aon. However, it is possible that future results of operations or cash flows for any particular quarterly or annual period could be materially affected by an unfavorable resolution of these matters.

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Our success depends, in part, on our ability to attract and retain experienced and qualified personnel.

        Our future success depends on our ability to attract and retain experienced personnel, including underwriters, brokers and other professional personnel. Competition for such experienced professional personnel is intense. If we cannot hire and retain talented personnel, our business, operating results and financial condition could be adversely affected.

        In September 2004, we announced that we were implementing a management succession plan and began a search for a new chief executive officer. Patrick G. Ryan, our current chief executive officer and chairman of the Board of Directors, will continue to serve as chief executive officer until a successor is found.

        We are actively seeking a new chief executive officer, but we can provide no assurances as to when an executive will be hired to fill this management position. In addition, we cannot give assurance that this search and any related uncertainty regarding the future composition of our management team will not adversely impact our results of operations. Once we hire a chief executive officer, our business may be impacted by our ability to successfully integrate him or her, his or her familiarity with our business, his or her ability to develop relationships with our employees and his or her implementation of new business strategies.

We are subject to increasing costs arising from errors and omissions claims against us.

        We have experienced an increase in the frequency and severity of errors and omissions claims against us, which has and may continue to substantially increase our expenses. In our insurance brokerage business, we often assist our clients with matters which include the placement of insurance coverage and the handling of related claims. Errors and omissions claims against us may allege our potential liability for all or part of the amounts in question. Errors and omissions claims could include, for example, the failure of our employees or sub-agents, whether negligently or intentionally, to place coverage correctly or notify claims on behalf of clients or to provide insurance carriers with complete and accurate information relating to the risks being insured. It is not always possible to prevent and detect errors and omissions, and the precautions we take may not be effective in all cases. In addition, errors and omissions claims may harm our reputation or divert management resources away from operating our business.

Our businesses are subject to extensive governmental regulation which could reduce our profitability or limit our growth.

        Our businesses are subject to extensive federal, state and foreign governmental regulation and supervision, which could reduce our profitability or limit our growth by increasing the costs of regulatory compliance, limiting or restricting the products or services we sell or the methods by which we sell our products and services or subjecting our businesses to the possibility of regulatory actions or proceedings. With respect to our insurance brokerage businesses, this supervision generally includes the licensing of insurance brokers and agents and third-party administrators and the regulation of the handling and investment of client funds held in a fiduciary capacity. Our continuing ability to provide insurance brokering and third-party administration in the jurisdictions in which we currently operate depends on our compliance with the rules and regulations promulgated from time to time by the regulatory authorities in each of these jurisdictions. Also, we can be affected indirectly by the governmental regulation and supervision of other insurance companies. For instance, if we are providing managing general underwriting services for an insurer we may have to contend with regulations affecting our client. Further, regulation affecting the insurance companies with whom our brokers place business can affect how we conduct those operations.

        Most insurance regulations are designed to protect the interests of policyholders rather than stockholders and other investors. In the United States, this system of regulation, generally administered by a department of insurance in each state in which we do business, affects the way we can conduct our

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insurance underwriting business. Furthermore, state insurance departments conduct periodic examinations of the affairs of insurance companies and require the filing of annual and other reports relating to the financial condition of insurance companies, holding company issues and other matters.

        Although the federal government does not directly regulate the insurance business, federal legislation and administrative policies in several areas, including employee benefit plan regulation, age, race, disability and sex discrimination, investment company regulation, financial services regulation, securities laws and federal taxation, do affect the insurance industry generally and our insurance underwriting subsidiaries in particular. With respect to our international operations, we are subject to various regulations relating to, among other things, licensing, currency, policy language and terms, reserves and the amount of local investment. These various regulations also add to our cost of doing business through increased compliance expenses, the financial impact of use of capital restrictions and increased training and employee expenses. Furthermore, the loss of a license in a particular jurisdiction could restrict or eliminate our ability to conduct business in that jurisdiction.

        In all jurisdictions the applicable laws and regulations are subject to amendment or interpretation by regulatory authorities. Generally, such authorities are vested with relatively broad discretion to grant, renew and revoke licenses and approvals, and to implement regulations. Accordingly, we may be precluded or temporarily suspended from carrying on some or all of our activities or otherwise fined or penalized in a given jurisdiction. No assurances can be given that our businesses can continue to be conducted in any given jurisdiction as they have been in the past.

Our significant global operations expose us to various international risks that could adversely affect our business.

        A significant portion of our operations is conducted outside the United States. Accordingly, we are subject to legal, economic and market risks associated with operating in foreign countries, including:

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the general economic and political conditions existing in those countries;



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imposition of limitations on conversion of foreign currencies or remittance of dividends and other payments by foreign subsidiaries;



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imposition or increase of withholding and other taxes on remittances and other payments by subsidiaries;



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hyperinflation in certain foreign countries;



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imposition or increase of investment and other restrictions by foreign governments;



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longer payment cycles;



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greater difficulties in accounts receivables collection; and



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the requirement of complying with a wide variety of foreign laws.

        Some of our foreign brokerage subsidiaries receive revenues in currencies that differ from their functional currencies. We must also translate the financial results of our foreign subsidiaries into United States dollars. Although we use various derivative financial instruments to help protect against adverse transaction and translation effects due to exchange rate fluctuations, we cannot eliminate such risks, and significant changes in exchange rates may adversely affect our results.

Our financial results could be adversely affected if assumptions used in establishing our underwriting reserves differ from actual experience.

        We maintain reserves as an estimate of our liability under insurance policies issued by our insurance underwriting subsidiaries. The reserves that we maintain that could cause variability in our financial results consist of (1) unearned premium reserves, (2) policy and contract claim reserves, and

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(3) future policy benefit reserves. Unearned premium reserves generally reflect our liability to return premiums we have collected under policies in the event of the lapse or cancellation of those policies. Under U.S. generally accepted accounting principles, premiums we have collected generally become "earned" over the life of a policy by means of a reduction in the amount of the unearned premium reserve associated with the policy. Unearned premium reserves are particularly significant with respect to our warranty business, given that the premiums we receive for warranty products generally cover an extended period of time. If there are significant lapses or cancellations of these types of policies, or expected losses for existing policies develop adversely and therefore premiums are not earned as expected, it may be necessary to accelerate the amortization of deferred policy acquisition expenses associated with the policies, because these deferred expenses are amortized over the projected life of the policies, or establish additional reserves to cover premium deficiencies.

        Policy and contract claim reserves reflect our estimated liability for unpaid claims and claims adjustment expenses, including legal and other fees and general expenses for administering the claims adjustment process, and for reported and unreported losses incurred as of the end of each accounting period. If the reserves originally established for future claims prove inadequate, we would be required to increase our liabilities, which could have an adverse effect on our business, results of operations and financial condition.

        The obligation for policy and contract claims does not represent an exact calculation of liability. Rather, reserves represent our best estimate of what we expect the ultimate settlement and administration of claims will cost. These estimates represent informed judgments based on our assessment of currently available data, as well as estimates of future trends in claims severity, frequency, judicial theories of liability and other factors. Many of these factors are not quantifiable in advance and both internal and external events, such as changes in claims handling procedures, inflation, judicial and legal developments and legislative changes, can cause our estimates to vary. The inherent uncertainty of estimating reserves is greater for certain types of liabilities, where the variables affecting these types of claims are subject to change and long periods of time may elapse before a definitive determination of liability is made. Reserve estimates are periodically refined as experience develops and further losses are reported and settled. Adjustments to reserves are reflected in the results of the periods in which such estimates are changed. Because setting the level of reserves for policy and contract claims is inherently uncertain, we cannot assure that our current reserves will prove adequate in light of subsequent events.

        Future policy benefit reserves generally reflect our liability to provide future life insurance benefits and future accident and health insurance benefits on guaranteed renewable and non-cancelable policies. Future policy benefit reserves on accident and health and life products have been provided on the net level premium method. These reserves are calculated based on assumptions as to investment yield, mortality, morbidity and withdrawal rates that were determined at the date of issue and provide for possible adverse deviations.

The perceived conflicts associated with our insurance brokerage and underwriting businesses could limit our growth.

        Historically, we have not been able to take advantage of certain business opportunities due to the perceived conflicts associated with owning both our insurance brokerage and underwriting businesses. For example, we have generally refrained from offering our extended warranty products and services through competing insurance brokers. Independent brokers have been reluctant to do business with our insurance underwriting business because they believed that any fees or information provided to us would ultimately benefit our competing brokerage business. These brokers also have been concerned that any information gleaned by our underwriting business regarding their clients and their clients' insurance needs would be shared with our competing brokerage business to solicit new business from these clients. Similarly, competing underwriters have feared that our brokers could share information

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with our underwriting business in an effort to help secure desirable business or, alternatively, seek price quotes from them only for undesirable business. In the future, these perceived conflicts could limit our ability to expand our product and service offerings and seek new business through independent brokerage channels.

Each of our business lines may be adversely affected by an overall decline in economic activity.

        The demand for property and casualty insurance generally rises as the overall level of economic activity increases and generally falls as such activity decreases, affecting both the commissions and fees generated by our brokerage and consulting businesses and the premiums generated by our underwriting businesses. In particular, a growing number of insolvencies associated with an economic downturn, especially insolvencies in the insurance industry, could adversely affect our brokerage business through the loss of clients or by hampering our ability to place insurance and reinsurance business. Moreover, the results of our consulting business are generally affected by the level of business activity of our clients, which in turn is affected by the level of economic activity in the industries and markets these clients serve. As our clients become adversely affected by declining business conditions, they may choose to delay or forgo consulting engagements with us.

Recent and proposed accounting rule changes could negatively affect our financial position and results.

        Recent accounting changes effected and proposals made could negatively affect our financial position and results of operations. In December 2004, the FASB issued Statement No. 123 (revised 2004) Share-Based Payment. Statement No. 123(R) is a revision of Statement No. 123, and supersedes APB Opinion No. 25. Statement No. 123(R) establishes standards for the accounting of transactions in which an entity exchanges its equity instruments for goods or services. It also addresses transactions in which an entity incurs liabilities in exchange for goods or services that are based on the fair value of the entity's equity instruments or that may be settled by the issuance of those equity instruments.

        Currently we follows APB No. 25 for share-based payments to employees. As such, we expense the cost of stock awards over the period during which an employee is required to provide service in exchange for the award. We currently do not recognize compensation expense for stock options. Statement No. 123 (R) will require us to recognize compensation expense, beginning in the third quarter 2005, for the unvested portion of compensation expense related to stock options issued before July 1, 2005. The expense will be recognized over the remaining vesting period of the options. We will continue to recognize compensation expense for stock awards issued before July 1, 2005.

        All stock awards and options issued after July 1, 2005 will be accounted for under the rules of Statement No. 123(R). The Statement requires us to measure the cost of employee services received in exchange for an award of equity instruments based on the grant-date fair value of the award (with limited exceptions). The cost will then be recognized over the period during which an employee is required to provide service in exchange for the award. No compensation expense is recognized for equity instruments for which employees do not render the requisite service. The Statement allows companies to choose among various valuation techniques to estimate the fair value at the grant date of employee stock options and similar instruments.

        We are currently evaluating the Statement's transition methods and do not expect this Statement to have an effect materially different than that of the pro forma Statement No. 123 disclosures provided in Note 1 to our consolidated financial statements.

We have substantial debt outstanding that could adversely affect our financial flexibility.

        We have substantial debt outstanding. As of December 31, 2004, we had total consolidated debt outstanding, including our redeemable preferred stock, of approximately $2.2 billion. This substantial amount of debt outstanding could adversely affect our financial flexibility.

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We are a holding company and, therefore, may not be able to receive dividends in needed amounts from our subsidiaries.

        Our principal assets are the shares of capital stock of our subsidiaries, including our insurance underwriting companies. We have to rely on dividends from these subsidiaries to meet our obligations for paying principal and interest on outstanding debt obligations and for paying dividends to stockholders and corporate expenses. Payments from our underwriting subsidiaries are limited by governmental regulation and depend on the surplus and future earnings of these subsidiaries. In some circumstances, specific payments from our insurance underwriting subsidiaries may require prior regulatory approval, and we may not be able to receive dividends from these subsidiaries at times and in the amounts we anticipate or require.

The volume of premiums we write and our profitability are affected by the availability of reinsurance and the size and adequacy of our insurance company subsidiaries' capital base.

        The level of business that our insurance underwriting subsidiaries are able to write depends on the size and adequacy of their capital base. Many state insurance laws to which they are subject impose risk-based capital requirements for purposes of regulating insurer solvency. Insurers having less statutory surplus than that required by the risk-based capital model formula generally are subject to varying degrees of regulatory scrutiny and intervention depending on the level of capital inadequacy. As of December 31, 2004, each of our insurance company subsidiaries exceeds NAIC risk-based statutory surplus requirements.

        We purchase reinsurance for certain of the risks underwritten by our insurance company subsidiaries. Market conditions beyond our control determine the availability and cost of the reinsurance protection we purchase, which may affect the level of business we are able to write and our profitability. We cannot assure that we will be able to maintain our current reinsurance facilities or that we can obtain other reinsurance facilities in adequate amounts and at favorable rates. If we are unable to renew our expiring facilities or to obtain new reinsurance facilities, either our net exposures would increase or, if we are unwilling to bear an increase in net exposures, we would have to reduce the level of our underwriting commitments. Either of these potential developments could adversely affect our underwriting business.

We cannot guarantee that our reinsurers will pay in a timely fashion, if at all.

        To better manage our portfolio of underwriting risk, we purchase reinsurance by transferring part of the risk that we assume (known as ceding) to a reinsurance company in exchange for part of the premium that we receive in connection with the risk. Although reinsurance makes the reinsurer liable to us to the extent the risk is transferred (or ceded) to the reinsurer, it does not relieve us of our liability to our policyholders. Accordingly, we bear credit risk with respect to our reinsurers. Recently, due to industry and general economic conditions, there is an increasing risk of insolvency among reinsurance companies, resulting in a greater incidence of litigation and affecting the recoverability of claims. We cannot assure that our reinsurers will pay the reinsurance recoverables owed to us or that they will pay these recoverables on a timely basis.

Information Concerning Forward-looking Statements

        This report contains certain statements related to future results, or states our intentions, beliefs and expectations or predictions for the future which are forward-looking statements as that term is defined in the Private Securities Litigation Reform Act of 1995. These forward-looking statements are subject to certain risks and uncertainties that could cause actual results to differ materially from either historical or anticipated results depending on a variety of factors. Potential factors that could impact results include: general economic conditions in different countries in which we do business around the world, changes in global equity and fixed income markets that could affect the return on invested

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assets, fluctuations in exchange and interest rates that could influence revenue and expense, rating agency actions that could affect our ability to borrow funds, funding of our various pension plans, changes in the competitive environment, changes in commercial property and casualty markets and commercial premium rates that could impact revenues, changes in revenues and earnings due to the elimination of contingent commissions, other uncertainties surrounding a new compensation model, the impact of regulatory investigations brought by state attorneys general and state insurance regulators related to our compensation arrangements with underwriters and related issues, the impact of class actions and individual lawsuits including derivative actions and claims under ERISA, the cost of resolution of other contingent liabilities and loss contingencies, the difference in ultimate paid claims in our underwriting companies from actuarial estimates, and other factors disclosed under "Risk Factors" elsewhere in this document.

Website Access to Reports and Other Information

        Our annual report on Form 10-K, quarterly reports on Form 10-Q, current reports on Form 8-K and all amendments to those reports are made available free of charge through our website (http://www.aon.com) as soon as practicable after such material is electronically filed with or furnished to the Securities and Exchange Commission. Also posted on our website, and available in print upon request, are the charters for our Audit Committee, Organization and Compensation Committee, Governance/Nominating Committee and Investment Committee, our Governance Guidelines, our Code of Ethics and our Code of Ethics for Senior Financial Officers. Within the time period required by the SEC and the New York Stock Exchange, we will post on our website any amendment to or waiver of the Code of Ethics for Senior Financial Officers, as well as any amendment to the Code of Ethics or waiver thereto applicable to any executive officer or director. The information provided on our website is not part of this report, and is therefore not incorporated herein by reference.

Item 2.    Properties.

        Our business activities are conducted principally in leased office space in cities throughout the world. Certain of our subsidiaries do own and occupy a few office buildings in the U.S. and certain foreign countries. In general, no difficulty is anticipated in negotiating renewals as leases expire or in finding other satisfactory space if the premises become unavailable. In certain circumstances, we may have unused space and may seek to sublet such space to third parties, depending upon the demands for office space in the locations involved.

Item 3.    Legal Proceedings.

        We hereby incorporate by reference Note 14, "Contingencies," of the Notes to Consolidated Financial Statements in Part II, Item 8 of this report.

Item 4.    Submission of Matters to a Vote of Security Holders.

        None.

Executive Officers of the Registrant

        Our executive officers are regularly elected by our Board of Directors at the annual meeting of the Board which is held following each annual meeting of our stockholders. Our executive officers were elected to their current positions on May 21, 2004 to serve until the meeting of the Board following the

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annual meeting of stockholders to be held on May 20, 2005. Ages shown for executive officers are as of December 31, 2004.

        Information concerning Mr. Ryan is incorporated by reference from the disclosure set forth under the heading "Election of Directors" in our Proxy Statement for the 2005 Annual Meeting of Stockholders to be held on May 20, 2005.

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PART II

Item 5.    Market for Registrant's Common Equity, Related Stockholder Matters and Issuer Repurchases of Equity Securities.

        Aon's common stock, par value $1.00 per share, is traded on the New York Stock Exchange. We hereby incorporate by reference the "Dividends paid per share" and "Price range" data under the heading "Quarterly Financial Data" in Part II, Item 8 of this report.

        Aon had approximately 11,231 holders of record of its common stock as of February 28, 2005.

        We hereby incorporate by reference Note 10, "Redeemable Preferred Stock, Capital Securities and Stockholders' Equity" of the Notes to Consolidated Financial Statements in Part II, Item 8 of this report.

        During the fourth quarter ended December 31, 2004, no purchases of Aon's common stock were made by or on behalf of Aon or any "affiliated purchaser" (as defined in Rule 10b-18(a)(3) under the Securities Exchange Act of 1934).

        Information relating to the compensation plans under which equity securities of Aon are authorized for issuance is set forth under Part III, Item 12 of this report and is incorporated herein by reference.

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Item 6.    Selected Financial Data.

Selected Financial Data