As filed with the Securities and Exchange Commission on November 8, 2011

Registration No. 333-175091

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

Amendment No. 5 to
FORM S-1
REGISTRATION STATEMENT
UNDER
THE SECURITIES ACT OF 1933

Radius Health, Inc.
(Exact name of registrant as specified in its charter)

201 Broadway, 6th Floor. Cambridge, MA, 02139
(617) 551-4700
(Address, including zip code, and telephone number, including
area code, of registrant's principal executive offices)

C. Richard Lyttle, Ph.D.
Chief Executive Officer
Radius Health, Inc.
201 Broadway, 6th Floor
Cambridge, MA, 02139
(617) 551-4700
(Name, address, including zip code, and telephone number, including area code, of agent for service)

Copies to:
Julio E. Vega, Esq.
Matthew J. Cushing, Esq.
Bingham McCutchen LLP
One Federal Street
Boston, Massachusetts 02110
(617) 951-8000

Approximate date of commencement of proposed sale to the public:
Promptly after the effective date of this Registration Statement, subject to applicable contractual lock-up agreements.

         If any of the securities being registered on this Form are to be offered on a delayed or continuous basis pursuant to Rule 415 under the Securities Act of check the following box.    ý

         If this Form is filed to register additional securities for an offering pursuant to Rule 462(b) under the Securities Act, check the following box and list the Securities Act registration statement number of the earlier effective registration statement for the same offering.    o

         If this Form is a post-effective amendment filed pursuant to Rule 462(c) under the Securities Act, check the following box and list the Securities Act registration statement number of the earlier effective registration statement for the same offering.    o

         If this Form is a post-effective amendment filed pursuant to Rule 462(d) under the Securities Act, check the following box and list the Securities Act registration statement number of the earlier effective registration statement for the same offering.    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.

         THE REGISTRANT HEREBY AMENDS THIS REGISTRATION STATEMENT ON SUCH DATE OR DATES AS MAY BE NECESSARY TO DELAY ITS EFFECTIVE DATE UNTIL THE REGISTRANT SHALL FILE A FURTHER AMENDMENT WHICH SPECIFICALLY STATES THAT THIS REGISTRATION STATEMENT SHALL THEREAFTER BECOME EFFECTIVE IN ACCORDANCE WITH SECTION 8(A) OF THE SECURITIES ACT OF 1933 OR UNTIL THE REGISTRATION STATEMENT SHALL BECOME EFFECTIVE ON SUCH DATE AS THE SECURITIES AND EXCHANGE COMMISSION, ACTING PURSUANT TO SAID SECTION 8(A), MAY DETERMINE.

EXPLANATION OF AMENDMENT

        The purpose of this Amendment No. 5 to our Registration Statement on Form S-1 filed with the Securities Exchange Commission on June 23, 2011, and amended by Amendment No. 1 thereto, filed on September 1, 2011, Amendment No. 2 thereto, filed on October 6, 2011, Amendment No. 3 thereto, filed on October 28, 2011, and Amendment No. 4 thereto, filed on November 7, 2011 (as amended, the "Form S-1"), is to (a) to update the Exhibit Index to change certain cross-references and (b) to incorporate into the Form S-1 exhibits filed with Company's Amendment No. 6 to Form 8-K filed on November 9, 2011.

        No other changes have been made to the Form S-1 other than those described above. This Amendment No. 5 does not reflect any subsequent events occurring after the filing date of Amendment No. 4 to the Form S-1, other than described above, or modify or update in any way disclosures made in the prospectus included in the Form S-1 as amended by Amendment No. 4 thereto.

PART II

INFORMATION NOT REQUIRED IN PROSPECTUS

Exhibits and Financial Statement Schedules

(a)

Exhibits

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Undertakings

        The undersigned registrant hereby undertakes:

        (1)   To file, during any period in which offers or sales are being made, a post-effective amendment to this registration statement:

          (i)  To include any prospectus required by section 10(a)(3) of the Securities Act of 1933;

         (ii)  To reflect in the prospectus any facts or events arising after the effective date of the registration statement (or the most recent post-effective amendment thereof) which, individually or in the aggregate, represent a fundamental change in the information set forth in the registration statement. Notwithstanding the foregoing, any increase or decrease in volume of securities offered (if the total dollar value of securities offered would not exceed that which was registered) and any deviation from the low or high end of the estimated maximum offering range may be reflected in the form of prospectus filed with the Commission pursuant to Rule 424(b) (§ 230.424(b) of this chapter) if, in the aggregate, the changes in volume and price represent no more than 20% change in the maximum aggregate offering price set forth in the "Calculation of Registration Fee" table in the effective registration statement; and

        (iii)  To include any material information with respect to the plan of distribution not previously disclosed in the registration statement or any material change to such information in the registration statement.

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        (2)   That, for the purpose of determining any liability under the Securities Act of 1933, each such post-effective amendment shall be deemed to be a new registration statement relating to the securities offered therein, and the offering of such securities at that time shall be deemed to be the initial bona fide offering thereof.

        (3)   To remove from registration by means of a post-effective amendment any of the securities being registered which remain unsold at the termination of the offering.

        (4)   That, for the purpose of determining liability of the registrant under the Securities Act of 1933 to any purchaser in the initial distribution of the securities:

        The undersigned registrant undertakes that in a primary offering of securities of the undersigned registrant pursuant to this registration statement, regardless of the underwriting method used to sell the securities to the purchaser, if the securities are offered or sold to such purchaser by means of any of the following communications, the undersigned registrant will be a seller to the purchaser and will be considered to offer or sell such securities to such purchaser:

          (i)  Any preliminary prospectus or prospectus of the undersigned registrant relating to the offering required to be filed pursuant to Rule 424 (§ 230.424 of this chapter);

         (ii)  Any free writing prospectus relating to the offering prepared by or on behalf of the undersigned registrant or used or referred to by the undersigned registrant;

        (iii)  The portion of any other free writing prospectus relating to the offering containing material information about the undersigned registrant or its securities provided by or on behalf of the undersigned registrant; and

        (iv)  Any other communication that is an offer in the offering made by the undersigned registrant to the purchaser.

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SIGNATURES

        Pursuant to the requirements of the Securities Act of 1933, the registrant has duly caused this amendment no. 5 to registration statement to be signed on its behalf by the undersigned, thereunto duly authorized, in the City of Cambridge, The Commonwealth of Massachusetts on this 8^th day of November 2011.

SIGNATURES AND POWER OF ATTORNEY

        Pursuant to the requirements of the Securities Act of 1933, this statement has been signed by the following persons in the capacities indicated as of the 8^th day of November 2011.

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EXHIBIT INDEX

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