On December 13, 2012, Infinity Pharmaceuticals, Inc. ("we," "us" or "our") entered into an underwriting agreement (the "Underwriting Agreement") with Morgan Stanley & Co. LLC and J.P. Morgan Securities LLC acting as representatives of the underwriters named therein (collectively, the "Underwriters"), relating to an underwritten public offering of 5,696,923 shares (the "Shares") of our common stock, par value $0.001 per share (the "Common Stock"). All of the shares are being sold by us. The offering price to the public is $26.33 per share, and the Underwriters have agreed to purchase the Shares from us pursuant to the Underwriting Agreement at a price of $24.7502 per share. After underwriting discounts and commissions and estimated offering expenses, we expect to receive net proceeds of approximately $140.750 million (assuming the Underwriters' over-allotment option referenced below is not exercised). Under the terms of the Underwriting Agreement, we have granted the Underwriters an option, exercisable for 30 days, to purchase up to an additional 854,538 shares of Common Stock to cover over-allotments, if any, at the public offering price less the underwriting discounts and commissions.

The Shares will be issued pursuant to an automatic shelf registration statement the Company filed with the U.S. Securities and Exchange Commission (the "SEC"), (File No. 333-184775). A prospectus supplement relating to the offering has been filed with the SEC. The closing of the offering is expected to take place on December 18, 2012, subject to the satisfaction of customary closing conditions.

A copy of the Underwriting Agreement is attached as Exhibit 1.1 hereto and is incorporated herein by reference. The foregoing description of the material terms of the Underwriting Agreement does not purport to be complete and is qualified in its entirety by reference to such exhibit.

A copy of the legal opinion and consent of Wilmer Cutler Pickering Hale and Dorr LLP relating to the Shares is attached as Exhibit 5.1 hereto.

We issued a press release on December 13, 2012 announcing the pricing of the public offering. This press release is attached as Exhibit 99.1 hereto and is incorporated herein by reference.

(d) The following exhibits are included in this report:

Exhibit
  No.                                     Description

 1.1         Underwriting Agreement, dated December 13, 2012, between the Company
             and Morgan Stanley & Co. LLC and J.P. Morgan Securities LLC, as
             representatives of the underwriters named therein.

 5.1         Opinion of Wilmer Cutler Pickering Hale and Dorr LLP.

23.1         Consent of Wilmer Cutler Pickering Hale and Dorr LLP (included in
             Exhibit 5.1).

99.1         Press Release, dated December 13, 2012.