Exhibit 99.1

For immediate release

Endeavour Announces First Quarter 2011
Financial and Operational Results

Houston, May 4, 2011 — Endeavour International Corporation (NYSE: END) (LSE: ENDV) today reported adjusted EBITDA for the first quarter of 2011 was $4.1 million compared to $9.7 million in the first quarter of 2010. On a GAAP basis, net loss was $(7.5) million for the first quarter of 2011 as compared to $(15.2) million for the same quarter in 2010.

“Since the beginning of the year, Endeavour has achieved several major milestones with respect to its development programs in the United Kingdom and continued to progress its growing U.S. portfolio. The deployment of infrastructure for the Bacchus oil field moves the project closer to first production this year while our agreement on the commercial terms for the Rochelle development allows us to begin construction of the production facilities necessary to bring additional gas production on line in 2012,” said William L. Transier, chairman, chief executive officer and president. “The successful conclusion of our equity offering during the quarter and the subsequent redemption of our 6% Senior Notes is a significant step in our financial plan, positioning the company to further simplify its capital structure and deliver production growth in 2011 and beyond.”

Highlights since the beginning of the year include:

Executed a Successful Equity Offering and Redeemed the 6% Senior Notes — The Company completed a public equity offering of 11.5 million shares of its common stock at a price of $11.00 per share for total proceeds of $116.8 million, net of underwriting fees and expenses. The offering closed on March 29, 2011. Endeavour used a portion of the net proceeds from the offering to redeem all $81.25 million of its 6% Convertible Senior Notes due 2012 on April 20, 2011.

Achieved Field Development Plan Approval and Agreed to Commercial Terms with the Scott Platform Owners for Rochelle Production — Endeavour, operator of the Rochelle field, announced the agreement of commercial terms for the processing and transportation of East Rochelle production on the Scott Platform in the Central North Sea. The Field Development Plan for East Rochelle calling for the subsea development to be linked by a 30-kilometer pipeline to production facilities on the Scott platform was approved by the United Kingdom’s Department of Energy and Climate Change in February. First production from East Rochelle is currently planned for the second half of 2012.

Continuing Progress of Bacchus Field Development — The installation of a 6.5 kilometer pipeline connecting the Bacchus field to the Forties Alpha platform was successfully completed during the quarter. The rig that will drill the three production wells is expected to move, from its current position in the Forties Field, to the Bacchus location in May. Production from the field is currently expected to commence during the third quarter with a ramp up to peak production toward the end of the year.

On-going Drilling Activity in the U.S. Shale Plays — Endeavour drilled five wells in its Haynesville and Cotton Valley plays located in Louisiana and East Texas, respectively. These wells are expected to be completed and brought on production in the next two months, as the Company continues to maintain a two-rig program in the area. In the Pennsylvania Marcellus, Endeavour successfully drilled and cased two horizontal wells in Cameron County. These wells will be completed later this year while efforts continue to expand the existing pipeline gathering infrastructure.

Amended the Terms of the 11.5% Convertible Bonds — The Company amended the terms of its 11.5% Convertible Bonds due 2014. The maturity date for the securities was extended from January 2014 to January 2016 and the interest rate payable after March 2014 was reduced from 11.5% to 7.5%. The amendment also moves the date at which the bond holders have the right to demand first payment and the date at which the conversion price could be reset to market from the original date of January 2012 to the maturity date of January 2016.

Completed Transfer from the NYSE Amex to the NYSE Main Market — Endeavour completed its transfer from the NYSE Amex to the NYSE Main Market on March 15, 2011. The Company continues to trade under the ticker symbol “END” in the United States.

Earnings Conference Call, Wednesday, May 4, 2011 at 9:00 a.m., Central Standard Time, 3:00 p.m. British Summer Time

Endeavour International will host a conference call and web cast to discuss its 2011 first quarter financial and operating results on Wednesday, May 4, 2011 at 9 a.m. Central Standard Time, 3 p.m. British Summer Time. To participate and ask questions during the conference call, dial the local country telephone number and the confirmation code 5667933. The toll-free numbers are 800-776-0087 in the United States and 0-808-101-1402 in the United Kingdom. Other international callers should dial 913-312-1301 (tolls apply). To listen only to the live audio web cast access Endeavour’s home page at http://www.endeavourcorp.com. A replay will be available beginning at 12:20 p.m. Central Standard Time on May 4 through 12:20 p.m. on May 11 by dialing toll free 888-203-1112 (U.S.) or 719-457-0820 (international), confirmation code 5667933.

Endeavour International Corporation is an oil and gas exploration and production company focused on the acquisition, exploration and development of energy reserves in the North Sea and the United States. For more information, visit http://endeavourcorp.com.

Additional information for investors:

Certain statements in this news release should be regarded as “forward-looking” statements within the meaning of the securities laws. These statements speak only as of the date made. Such statements are subject to assumptions, risk and uncertainty. Actual results or events may vary materially.

As of January 1, 2010, the Securities and Exchange Commission (SEC) changed its rules to permit oil and gas companies, in their filings with the SEC, to disclose not only proved reserves, but also probable reserves and possible reserves. Proved oil and gas reserves are those quantities of oil and gas, which, by analysis of geosciences and engineering data, can be estimated with reasonable certainty to be economically producible — from a given date forward, from known reservoirs, and under existing economic conditions, operating methods, and government regulations — prior to the time at which contracts providing the right to operate expire. Probable reserves include those additional reserves that a company believes are as likely as not to be recovered and possible reserves include those additional reserves that are less certain to be recovered than probable reserves. We use may use certain terms in our news releases, such as “reserve potential,” that the SEC’s guidelines strictly prohibit us from including in filings with the SEC. In addition, we do not represent that the probable or possible reserves described herein meet the recoverability thresholds established by the SEC in its new definitions. Investors are urged to also consider closely the disclosure in our filings with the SEC, available from our website at www.endeavourcorp.com. Endeavour is also subject to the requirements of the London Stock Exchange and considers the disclosures in this release to be appropriate and/or required under the guidelines of that exchange.

For further information:

Endeavour — Investor Relations
Mike Kirksey		713.307.8788
Darcey Matthews		713.307.8711
Pelham Public Relations — UK Media
Philip Dennis		+44 (0)207 861 3919
Henry Lerwill		+44 (0)207 861 3169

Endeavour International Corporation
Condensed Consolidated Balance Sheets
(Unaudited)
(Amounts in thousands)

		March 31,				December 31,
		2011				2010
Assets
Current Assets:
Cash and cash equivalents		$	152,146			$	99,267
Restricted cash			33,072				31,776
Accounts receivable			6,922				8,068
Prepaid expenses and other current assets			6,547				8,718
Total Current Assets			198,687				147,829
Property and Equipment, Net			444,728				364,677
Goodwill			211,886				211,886
Other Assets			22,256				25,895
Total Assets		$	877,557			$	750,287
Liabilities and Stockholders’ Equity
Current Liabilities:
Accounts payable		$	45,138			$	32,442
Current maturities of debt			100,350				21,600
Accrued expenses and other			20,564				22,642
Total Current Liabilities			166,052				76,684
Long-Term Debt			244,152				323,706
Deferred Taxes			92,604				77,200
Other Liabilities			56,839				64,927
Total Liabilities			559,647				542,517
Commitments and Contingencies
Series C Convertible Preferred Stock			53,152				53,152
Stockholders’ Equity			264,758				154,618
Total Liabilities and Stockholders’ Equity		$	877,557			$	750,287

Endeavour International Corporation
Condensed Consolidated Statement of Operations
(Unaudited)
(Amounts in thousands, except per share data)

		Three Months Ended
		March 31,
		2011				2010
Revenues		$	14,104			$	13,721
Cost of Operations:
Operating expenses			5,041				2,821
Depreciation, depletion and amortization			6,323				5,681
Impairment of oil and gas properties			—				7,692
General and administrative			4,713				4,431
Total Expenses			16,077				20,625
Loss From Operations			(1,973	)			(6,904	)
Other Income (Expense):
Derivatives:
Realized gains			—				232
Unrealized gains (losses)			4,464				(1,072	)
Interest expense			(12,523	)			(5,636	)
Interest income and other			(139	)			3,024
Total Other Expense			(8,198	)			(3,452	)
Loss Before Income Taxes			(10,171	)			(10,356	)
Income Tax Expense (Benefit)			(2,714	)			4,834
Net Loss			(7,457	)			(15,190	)
Preferred Stock Dividends			545				589
Net Loss to Common Stockholders		$	(8,002	)		$	(15,779	)
Net Loss per Common Share — Basic and Diluted		$	(0.30	)		$	(0.74	)
Weighted Average Number of Common Shares Outstanding:
Basic and Diluted			26,750				21,364

Endeavour International Corporation
Condensed Consolidated Statement of Cash Flows
(Unaudited)
(Amounts in thousands)

		Three Months Ended March 31,
		2011				2010
Cash Flows from Operating Activities:
Net loss		$	(7,457	)		$	(15,190	)
Adjustments to reconcile net loss to net cash provided by (used in) operating activities:
Depreciation, depletion and amortization			6,323				5,681
Impairment of oil and gas properties			—				7,692
Deferred tax expense (benefit)			(2,182	)			3,477
Unrealized (gains) losses on derivatives			(4,464	)			1,072
Amortization of non-cash compensation			875				1,153
Amortization of loan costs and discount			3,916				1,528
Non-cash interest expense			3,031				1,683
Other			2,602				(3,023	)
Changes in operating assets and liabilities			(18,066	)			5,341
Net Cash Provided by (Used in) Operating Activities			(15,422	)			9,414
Cash Flows From Investing Activities:
Capital expenditures			(20,148	)			(21,768	)
Acquisitions			(20,964	)			(29,407	)
(Increase) decrease in restricted cash			(1,295	)			2,859
Net Cash Used in Investing Activities			(42,407	)			(48,316	)
Cash Flows From Financing Activities:
(Repayments) borrowings under debt agreements			(5,400	)			25,000
Proceeds from issuance of common stock			116,822				20,011
Dividends paid			(506	)			(550	)
Other financing			(208	)			(923	)
Net Cash Provided by Financing Activities			110,708				43,538
Net Increase in Cash and Cash Equivalents			52,879				4,636
Cash and Cash Equivalents, Beginning of Period			99,267				27,287
Cash and Cash Equivalents, End of Period		$	152,146			$	31,923

Endeavour International Corporation
Operating Statistics
(Unaudited)

		Three Months Ended
		March 31,
		2011				2010
Sales volume (1)
Oil and condensate sales (Mbbls):
United Kingdom			100				113
United States			2				1
Total			102				114
Gas sales (MMcf):
United Kingdom			44				766
United States			965				280
Total			1,009				1,046
Oil equivalent sales (MBOE):
United Kingdom			108				241
United States			162				48
Total			270				289
Total BOE per day			3,001				3,209
Physical production volume (BOE per day): (2)
United Kingdom			1,307				3,118
United States			1,885				578
Total			3,192				3,696
Realized Prices (3)
Oil and condensate price ($  per Bbl):
Before commodity derivatives		$	98.79			$	70.24
Effect of commodity derivatives			—				(7.50	)
Realized prices including commodity derivatives		$	98.79			$	62.74
Gas price ($  per Mcf):
Before commodity derivatives		$	4.00			$	5.43
Effect of commodity derivatives			—				1.25
Realized prices including commodity derivatives		$	4.00			$	6.68
Equivalent oil price ($  per BOE):
Before commodity derivatives		$	52.22			$	47.51
Effect of commodity derivatives			—				1.55
Realized prices including commodity derivatives		$	52.22			$	49.06

Endeavour International Corporation
Reconciliation of GAAP to Non-GAAP Measures
(Unaudited)
(Amounts in thousands)

As required under Regulation G of the Securities Exchange Act of 1934, provided below are reconciliations of net income (loss) to the following non-GAAP financial measures: net income, as adjusted and Adjusted EBITDA. We use these non-GAAP measures as key metrics for our management and to demonstrate our ability to internally fund capital expenditures and service debt. The non-GAAP measures are useful in comparisons of oil and gas exploration and production companies as they exclude non-operating fluctuations in assets and liabilities.

		Three Months Ended
		March 31,
		2011				2010
Net loss		$	(7,457	)		$	(15,190	)
Impairment of oil and gas properties (net of tax) (1)			—				7,692
Unrealized (gain) loss on derivatives (net of tax) (2)			(5,469	)			1,174
Currency impact on deferred taxes			—				(59	)
Net Loss as Adjusted		$	(12,926	)		$	(6,383	)
Net loss		$	(7,457	)		$	(15,190	)
Unrealized (gain) loss on derivatives			(4,464	)			1,072
Net interest expense			12,418				5,633
Depreciation, depletion and amortization			6,323				5,681
Impairment of oil and gas properties			—				7,692
Income tax expense (benefit)			(2,714	)			4,834
Adjusted EBITDA		$	4,106			$	9,722