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

FORM 10-K/A
(Amendment No. 1)

Commission File Number 001-34620

IRONWOOD PHARMACEUTICALS, INC.
(Exact name of registrant as specified in its charter)

Registrant's telephone number, including area code: (617) 621-7722

Securities registered pursuant to Section 12(b) of the Act:

Securities registered pursuant to Section 12(g) of the Act: None

         Indicate by check mark if the registrant is a well-known seasoned issuer, as defined in Rule 405 of the Securities Act. Yes ý    No o

         Indicate by check mark if the registrant is not required to file reports pursuant to Section 13 or 15(d) of the Exchange Act. Yes o    No ý

         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 whether the Registrant has submitted electronically and posted on its corporate Web site, if any, every Interactive Data File required to be submitted and posted pursuant to Rule 405 of Regulation S-T during the preceding 12 months (or for such shorter period that the Registrant was required to submit and post such files). 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 a large accelerated filer, an accelerated filer, a non-accelerated filer or a smaller reporting company. See definitions of "large accelerated filer," "accelerated filer" and "smaller reporting company" in Rule 12b-2 of the Exchange Act.

         Indicate by check mark whether the registrant is a shell company (as defined in Rule 12b-2 of the Exchange Act). Yes o    No ý

         Aggregate market value of voting stock held by non-affiliates of the Registrant as of June 30, 2011: $1,375,964,667

         As of February 15, 2012, there were 75,186,090 shares of Class A common stock outstanding and 31,770,641 shares of Class B common stock outstanding.

DOCUMENTS INCORPORATED BY REFERENCE:

         Portions of the definitive proxy statement for our 2012 Annual Meeting of Stockholders are incorporated by reference into Part III of this report.

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EXPLANATORY NOTE

        We are filing this Amendment No. 1 to our Annual Report on Form 10-K for the year ended December 31, 2011 for the sole purpose of filing a corrected Exhibit 21.1—List of Registrant's Subsidiaries. The Exhibit 21.1 previously filed with our Annual Report on Form 10-K for the year ended December 31, 2011 was incorrectly included in our filing. The correct list of our subsidiaries is being filed herewith.

        This Amendment No. 1 is limited in scope to the foregoing, and should be read in conjunction with our Annual Report on Form 10-K filed earlier today.

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PART IV

Item 15.    Exhibits and Financial Statement Schedules

        (a)   List of documents filed as part of this report

(1)

Consolidated Financial Statements listed under Part II, Item 8 and included herein by reference.



(2)

Consolidated Financial Statement Schedules

No schedules are submitted because they are not applicable, not required or because the information is included in the Consolidated Financial Statements or Notes to Consolidated Financial Statements.

(3)

Exhibits

1

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2

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        (b)   Exhibits.

        The exhibits required by this Item are listed under Item 15(a)(3).

        (c)   Financial Statement Schedules.

        The financial statement schedules required by this Item are listed under Item 15(a)(2).

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SIGNATURES

        Pursuant to the requirements of Section 13 or 15(d) of the Securities Exchange Act of 1934, the registrant has duly caused this report to be signed on its behalf by the undersigned, thereunto duly authorized, in the City of Cambridge, Commonwealth of Massachusetts, on the 29th day of February 2012.

        Pursuant to the requirements of Section 13 or 15(d) of the Securities Exchange Act of 1934, this report has been signed below by the following persons on behalf of the registrant and in the capacities and on the date indicated.

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Exhibit Index

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