SEC FORM 3

SEC Form 3

FORM 3

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

INITIAL STATEMENT OF BENEFICIAL OWNERSHIP OF SECURITIES

Filed pursuant to Section 16(a) of the Securities Exchange Act of 1934
or Section 30(h) of the Investment Company Act of 1940

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1. Name and Address of Reporting Person^*

SILVERSTEIN JONATHAN

(Last)

(First)

(Middle)

C/O NXSTAGE MEDICAL, INC.

439 SOUTH UNION STREET, 5TH FLOOR

(Street)

LAWRENCE

01843

(City)

(State)

(Zip)

2. Date of Event Requiring Statement (Month/Day/Year)
07/23/2008

3. Issuer Name and Ticker or Trading Symbol
NxStage Medical, Inc. [ NXTM ]

4. Relationship of Reporting Person(s) to Issuer
(Check all applicable)

X	Director	X	10% Owner
	Officer (give title below)		Other (specify below)

5. If Amendment, Date of Original Filed (Month/Day/Year)

6. Individual or Joint/Group Filing (Check Applicable Line)

X	Form filed by One Reporting Person
	Form filed by More than One Reporting Person

Table I - Non-Derivative Securities Beneficially Owned
1. Title of Security (Instr. 4)	2. Amount of Securities Beneficially Owned (Instr. 4)	3. Ownership Form: Direct (D) or Indirect (I) (Instr. 5)	4. Nature of Indirect Beneficial Ownership (Instr. 5)
Common Stock	5,555,556	I	See Footnotes⁽¹⁾⁽²⁾

Table II - Derivative Securities Beneficially Owned (e.g., puts, calls, warrants, options, convertible securities)
1. Title of Derivative Security (Instr. 4)	2. Date Exercisable and Expiration Date (Month/Day/Year)		3. Title and Amount of Securities Underlying Derivative Security (Instr. 4)		4. Conversion or Exercise Price of Derivative Security	5. Ownership Form: Direct (D) or Indirect (I) (Instr. 5)	6. Nature of Indirect Beneficial Ownership (Instr. 5)
1. Title of Derivative Security (Instr. 4)	Date Exercisable	Expiration Date	Title	Amount or Number of Shares	4. Conversion or Exercise Price of Derivative Security	5. Ownership Form: Direct (D) or Indirect (I) (Instr. 5)	6. Nature of Indirect Beneficial Ownership (Instr. 5)
Warrant (right to buy)	05/23/2008	05/23/2013	Common Stock	1,111,111	$5.5	I	See Footnotes⁽³⁾

Explanation of Responses:

1. Includes 5,503,145 shares held directly by Caduceus Private Investments III, LP ("Caduceus") and 52,411 shares held directly by OrbiMed Associates III, LP ("OrbiMed Associates").

2. OrbiMed Advisors, LLC ("OrbiMed Advisors"), pursuant to its authority under its investment advisory contract with OrbiMed Associates, and OrbiMed Capital GP III LLC ("OrbiMed Capital"), pursuant to its authority as general partner of Caduceus, may be considered to hold indirectly 5,503,145 and 52,411 shares of Common Stock, respectively, and to have discretionary investment management authority over these shares. Such authority includes the power to vote and otherwise dispose of securities purchased by Orbimed Associates and Caduceus. As OrbiMed Advisors and OrbiMed Capital are under common control, Jonathan Silverstein, in his capacity as the general partner of OrbiMed Advisors, may be deemed to beneficially own these shares. Mr. Silverstein disclaims beneficial ownership of such shares except to the extent of his pecuniary interest therein.

3. Includes warrants to purchase 1,100,629 shares held directly by Caduceus and warrants to purchase 10,482 shares held directly by OrbiMed Associates. Pursuant to certain agreements and relationships described in further detail above, Jonathan Silverstein, in his capacity as the general partner of OrbiMed Advisors, LLC, may be deemed to beneficially own these shares. Mr. Silverstein disclaims beneficial ownership of such shares except to the extent of his pecuniary interest therein.

Remarks:

	/s/ Robert S. Brown, attorney-in-fact	07/24/2008
	** Signature of Reporting Person	Date
Reminder: Report on a separate line for each class of securities beneficially owned directly or indirectly.
* If the form is filed by more than one reporting person, see Instruction 5 (b)(v).
** Intentional misstatements or omissions of facts constitute Federal Criminal Violations See 18 U.S.C. 1001 and 15 U.S.C. 78ff(a).
Note: File three copies of this Form, one of which must be manually signed. If space is insufficient, see Instruction 6 for procedure.
Persons who respond to the collection of information contained in this form are not required to respond unless the form displays a currently valid OMB Number.