SEC Form 4

FORM 4		UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 STATEMENT OF CHANGES IN BENEFICIAL OWNERSHIP Filed pursuant to Section 16(a) of the Securities Exchange Act of 1934 or Section 30(h) of the Investment Company Act of 1940	OMB APPROVAL OMB Number: 3235-0287 Estimated average burden hours per response: 0.5
	Check this box if no longer subject to Section 16. Form 4 or Form 5 obligations may continue. See Instruction 1(b).

1. Name and Address of Reporting Person* ORBIMED ADVISORS LLC (Last) (First) (Middle) 767 3RD AVENUE 30TH FLOOR (Street) NEW YORK NY 10017 (City) (State) (Zip)	2. Issuer Name and Ticker or Trading Symbol NxStage Medical, Inc. [ NXTM ]	5. Relationship of Reporting Person(s) to Issuer (Check all applicable) X Director X 10% Owner Officer (give title below) Other (specify below)
	3. Date of Earliest Transaction (Month/Day/Year) 08/02/2010
	4. If Amendment, Date of Original Filed (Month/Day/Year)	6. Individual or Joint/Group Filing (Check Applicable Line) Form filed by One Reporting Person X Form filed by More than One Reporting Person

Table I - Non-Derivative Securities Acquired, Disposed of, or Beneficially Owned
1. Title of Security (Instr. 3)	2. Transaction Date (Month/Day/Year)	2A. Deemed Execution Date, if any (Month/Day/Year)	3. Transaction Code (Instr. 8)		4. Securities Acquired (A) or Disposed Of (D) (Instr. 3, 4 and 5)			5. Amount of Securities Beneficially Owned Following Reported Transaction(s) (Instr. 3 and 4)	6. Ownership Form: Direct (D) or Indirect (I) (Instr. 4)	7. Nature of Indirect Beneficial Ownership (Instr. 4)
			Code	V	Amount	(A) or (D)	Price
Common Stock	08/02/2010		S		17,400	D	$16.3851	5,000,300	I	See Footnotes(1)(2)(3)
Common Stock	08/03/2010		S		199,200	D	$16.4125	4,801,100	I	See Footnotes(2)(3)(4)
Common Stock	08/04/2010		S		101,100	D	$16.3084	4,700,000	I	See Footnotes(2)(3)(5)

Table II - Derivative Securities Acquired, Disposed of, or Beneficially Owned (e.g., puts, calls, warrants, options, convertible securities)
1. Title of Derivative Security (Instr. 3)	2. Conversion or Exercise Price of Derivative Security	3. Transaction Date (Month/Day/Year)	3A. Deemed Execution Date, if any (Month/Day/Year)	4. Transaction Code (Instr. 8)		5. Number of Derivative Securities Acquired (A) or Disposed of (D) (Instr. 3, 4 and 5)		6. Date Exercisable and Expiration Date (Month/Day/Year)		7. Title and Amount of Securities Underlying Derivative Security (Instr. 3 and 4)		8. Price of Derivative Security (Instr. 5)	9. Number of derivative Securities Beneficially Owned Following Reported Transaction(s) (Instr. 4)	10. Ownership Form: Direct (D) or Indirect (I) (Instr. 4)	11. Nature of Indirect Beneficial Ownership (Instr. 4)
				Code	V	(A)	(D)	Date Exercisable	Expiration Date	Title	Amount or Number of Shares

1. Name and Address of Reporting Person* ORBIMED ADVISORS LLC (Last) (First) (Middle) 767 3RD AVENUE 30TH FLOOR (Street) NEW YORK NY 10017 (City) (State) (Zip) Relationship of Reporting Person(s) to Issuer X Director X 10% Owner Officer (give title below) Other (specify below)

1. Name and Address of Reporting Person* ISALY SAMUEL D (Last) (First) (Middle) 767 3RD AVENUE 30TH FLOOR (Street) NEW YORK NY 10017 (City) (State) (Zip) Relationship of Reporting Person(s) to Issuer X Director X 10% Owner Officer (give title below) Other (specify below)

Explanation of Responses:

1. As of August 2, 2010, the Reporting Persons hold 5,000,300 shares of Common Stock, on behalf of other persons who have the right to receive or the power to direct the receipt of dividends or proceeds from the sale of such securities. Of this amount, Caduceus Private Investments III, LP ("Caduceus") and OrbiMed Associates III, LP ("Associates") hold 4,953,205 shares and 47,095 shares, respectively. OrbiMed Advisors LLC ("Advisors"), pursuant to its authority under its investment advisory contract with Associates, may be considered to hold indirectly 47,095 shares of Common Stock and OrbiMed Capital GP III LLC ("Capital"), pursuant to its authority as general partner of Caduceus, may be considered to hold indirectly 4,953,205 shares of Common Stock.

2. Advisors is a registered adviser under the Investment Advisers Act of 1940, as amended, that acts as an investment adviser to certain collective investment funds which hold shares of the Issuer. Samuel D. Isaly, a natural person, owns controlling interests in Advisors and Capital. Advisors acts as investment adviser to Associates. Capital acts as general partner to Caduceus. Pursuant to certain agreements and relationships, Advisors and Capital have discretionary investment management authority with respect to the assets of Associates and Caduceus. Such authority includes the power to vote and otherwise dispose of securities purchased by Associates and Caduceus. The Reporting Persons may be deemed to be a director of the issuer by virtue of having nominated a representative, now Jonathan T. Silverstein, to serve on the Issuer's board of directors.

3. The Reporting Persons disclaim beneficial ownership except to the extent of their pecuniary interest. This report shall not be deemed an admission that the Reporting Persons are beneficial owners for the purpose of Section 16 of the Securities Exchange Act of 1934, as amended, or for any other purpose.

4. As of August 3, 2010, the Reporting Persons hold 4,801,100 shares of Common Stock, on behalf of other persons who have the right to receive or the power to direct the receipt of dividends or proceeds from the sale of such securities. Of this amount, Caduceus and Associates hold 4,755,700 shares and 45,400 shares, respectively. Advisors, pursuant to its authority under its investment advisory contract with Associates, may be considered to hold indirectly 45,400 shares of Common Stock and Capital, pursuant to its authority as general partner of Caduceus, may be considered to hold indirectly 4,755,700 shares of Common Stock.

5. As of August 4, 2010, the Reporting Persons hold 4,700,000 shares of Common Stock, on behalf of other persons who have the right to receive or the power to direct the receipt of dividends or proceeds from the sale of such securities. Of this amount, Caduceus and Associates hold 4,655,600 shares and 44,400 shares, respectively. Advisors, pursuant to its authority under its investment advisory contract with Associates, may be considered to hold indirectly 44,400 shares of Common Stock and Capital, pursuant to its authority as general partner of Caduceus, may be considered to hold indirectly 4,655,600 shares of Common Stock.

	/s/ Samuel D. Isaly	08/04/2010
	** Signature of Reporting Person	Date
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* If the form is filed by more than one reporting person, see Instruction 4 (b)(v).
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