OFFER OF EMPLOYMENT

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Exhibit 10.17

[PRAECIS Letterhead]

October 20, 2004

Michael J. Keavany
10 Stoneybrook Lane
Malvern, PA 19355

Dear Michael,

        It is my pleasure to offer you the position of Senior Vice President, Sales and Marketing with PRAECIS PHARMACEUTICALS INCORPORATED, reporting directly to me. We are looking forward to having you join our management team.

        Base Pay —Your annual base salary will be $270,000, paid in 26 increments in the amount of $10,384.62 per pay period.

        Signing Payment —You will receive an on-hire signing payment in the amount of $55,000 to cover the loss of your 2004 incentive bonus from your previous employer.

        Management Bonus —You will be eligible to participate in the Company's Executive Management Bonus Plan (the "Bonus Plan"). The targeted value of your bonus award will be 30% of your annual base salary. Your actual bonus will be based upon the attainment of performance related objectives established annually and approved by the Board of Directors. Awards under the Bonus Plan are paid in cash.

        Stock Options —The Chairman of the Board will recommend to the Board of Directors an award of stock options to purchase 150,000 shares of PRAECIS common stock ("Common Stock"), at a per-share exercise price equal to the fair market value of the Common Stock on the date the stock option is granted, and in accordance with the Company's Third Amended and Restated 1995 Stock Plan (as it may be amended from time to time, the "Stock Plan"). These stock options will vest and become exercisable over a period of five years as follows: 20% of the total grant shall vest and become exercisable on the first anniversary of your date of hire; and 1.67% of the total grant will vest and become exercisable monthly thereafter for the subsequent 48 month-period, provided that you remain an employee of the Company at such times. Notwithstanding the foregoing, these stock options, and any additional stock options granted to you under the Stock Plan, shall automatically become fully vested and exercisable immediately in the event that your employment is terminated upon, or within a one-year period following, the effective date of a Change of Control (as defined below) of the Company, by (i) the Company, other than for Cause (as defined below), or (ii) you for Good Reason (as defined below). The stock option agreement(s) between you and the Company with respect to these options, and any additional stock options granted to you under the Stock Plan, will be in the Company's standard form, except that such stock options agreement(s) will provide for automatic acceleration of vesting and exercisability as set forth in the immediately preceding sentence, and will also provide that immediately upon a Change of Control, the section of each such stock option agreement captioned "No Exercise of Option if Employment Terminated for Misconduct" shall automatically cease to be of any force or effect and, accordingly, no termination of your employment with the Company upon or after a Change of Control will be, or will be deemed to be, a termination for "Misconduct" for purposes of such stock option agreement(s).

        Change of Control —If a Change of Control occurs and your employment with the Company is terminated upon, or within a one-year period following, the effective date of such Change of Control by (i) the Company, other than for Cause, or (ii) you for Good Reason, you shall be entitled to a lump sum payment equal to either (A) your annual salary for the year in which such Change of Control occurs, plus the full amount of your target award under the Bonus Plan for that year, if such termination occurs during the first twelve months of your employment with the Company, or (B) two times the sum of (x) your annual salary for the year in which such Change of Control occurs and (y) the full amount of your target award under the Bonus Plan for that year, if such termination occurs

at any time after the first anniversary of you employment with the Company. In addition, in the event of such termination, then for a period of either (i) one year following the effective date of such termination, if such termination occurs during the first twelve months of your employment with the Company, or (ii) two years following the effective date of termination, if such termination occurs at any time after the first anniversary of your commencement of employment with the Company, the Company shall, at its sole cost and expense, maintain in full force and effect for your benefit (and the benefit of your spouse and children, if applicable) the long-term disability and medical and dental insurance coverage maintained by the Company and as in effect immediately prior to the Change of Control or, if more favorable to you (and your spouse and children, if applicable), as in effect immediately prior to the occurrence of the first event or circumstance constituting Good Reason; provided that if the general terms and conditions of such insurance do not permit the continued coverage of you (and of your spouse and children, if applicable) as provided above, the Company shall, provide or arrange to provide you (and your spouse and children, if applicable) for the applicable period as provided above, at its sole cost and expense, with such insurance coverage having benefits substantially similar (with no reduction in benefits) to those which you (and you spouse and children, if applicable) would otherwise have been entitled had the continued coverage of you (and of your spouse and children, if applicable) as provided above been permitted under the general terms and conditions of such insurance.

        Withholding —The Company may withhold from any amounts payable hereunder such federal, state, local or foreign taxes as shall be required to be withheld pursuant to any applicable law or regulation.

        Parachute Tax Treatment —In the event that any payment or benefit you receive from the Company or an affiliate (collectively, the "Payments") would be subject to the excise tax (the "Excise Tax") imposed on "excess parachute payments" pursuant to Section 4999 of the Internal Revenue Code of 1986, as amended (the "Code"), then the Payments shall be reduced to the extent necessary so that no portion of the Payments is subject to the Excise Tax, provided that such reduction shall occur only if (A) the net amount of such Payments, as so reduced (and after subtracting the net amount of federal, state and local income taxes on such reduced Payments) is greater than (B) the net amount of such Payments without such reduction (but after subtracting the net amount of federal, state and local income taxes on such Payments and the amount of Excise Tax to which you would be subject in respect of such unreduced Payments). Determinations with respect to the preceding sentence shall be made by the Company's auditors which were serving as such prior to the Change of Control, in consultation with tax counsel selected by the Company.

        Certain Definitions —For the purposes of this agreement:

        A "Change of Control" shall occur or be deemed to have occurred if:

(i)

any individual, entity or "person" (within the meaning of Section 13(d)(3) or Section 14(d)(2) of the Securities Exchange Act of 1934, as amended (the "Exchange Act")), other than the Company, any trustee or other fiduciary holding securities under an employee benefit plan of the Company, or any corporation owned directly or indirectly by the stockholders of Company in substantially the same proportion as their ownership of stock of the Company, is or becomes the "beneficial owner" (as defined in Rule 13d-3 under the Exchange Act), directly or indirectly, of securities of the Company representing more than 50% of the combined voting power of the Company's then outstanding securities entitled to vote generally for the election of directors;

(ii)

individuals who, as of the date hereof, constitute the Company's Board of Directors (as of the date hereof, the "Incumben