SEC FORM 4

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

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)
08/21/2008

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)
1. Title of Security (Instr. 3)	2. Transaction Date (Month/Day/Year)	2A. Deemed Execution Date, if any (Month/Day/Year)	Code	V	Amount	(A) or (D)	Price		6. Ownership Form: Direct (D) or Indirect (I) (Instr. 4)	7. Nature of Indirect Beneficial Ownership (Instr. 4)
Common Stock	08/21/2008		P		26,100	A	$3.2503	5,581,656	I	See Footnotes⁽¹⁾⁽²⁾⁽³⁾
Common Stock	08/25/2008		P		108,900	A	$3.3711	5,690,556	I	See Footnotes⁽¹⁾⁽²⁾⁽³⁾

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)
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)	Code	V	(A)	(D)	Date Exercisable	Expiration Date	Title	Amount or Number of Shares	8. Price of Derivative Security (Instr. 5)		10. Ownership Form: Direct (D) or Indirect (I) (Instr. 4)	11. Nature of Indirect Beneficial Ownership (Instr. 4)
Warrant (right to buy)	$5.5	05/23/2008		P		1,111,111		05/23/2008	05/23/2013	Common Stock	1,111,111	⁽⁴⁾	1,111,111	I	See Footnotes⁽³⁾⁽⁵⁾
Director's Stock Option (right to buy)	$4.36	07/23/2008		A		11,667		07/23/2008	07/22/2013⁽⁸⁾	Common Stock	11,667	$0	11,667	I⁽⁶⁾	See Footnotes⁽³⁾⁽⁷⁾

1. Name and Address of Reporting Person^*

ORBIMED ADVISORS LLC

(Last)

(First)

(Middle)

767 3RD AVENUE

30TH FLOOR

(Street)

NEW YORK

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 THIRD AVENUE

30TH FLOOR

(Street)

NEW YORK

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 25, 2008, the Reporting Persons hold 5,690,556 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 5,636,745 and 53,811 shares, respectively. OrbiMed Advisors LLC ("Advisors"), pursuant to its authority under its investment advisory contract with Associates, may be considered to hold indirectly 53,811 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 5,636,745 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 Exchange Act of 1934, as amended, or for any other purpose.

4. For each share of Common Stock of the Issuer purchased on May 23, 2008, the purchaser also received a warrant to purchase .2 shares of Common Stock.

5. This amount represents (i) warrants to purchase 1,100,629 shares of Common Stock held by Caduceus, and (ii) warrants to purchase 10,482 shares of Common Stock held by Associates. Advisors, pursuant to its authority under its investment advisory contract with Associates, may be considered to hold indirectly warrants to purchase 10,482 shares of Common Stock and Capital, pursuant to its authority as general partner of Caduceus, may be considered to hold indirectly warrants to purchase 1,100,629 shares of Common Stock.

6. The amount shown represents (i) options to purchase 11,557 shares of Common Stock held by Caduceus, and (ii) options to purchase 110 shares of Common Stock held by Associates.

7. The options were granted directly to Mr. Silverstein, a director of the Issuer. Mr. Silverstein is also a partner of Advisors and Capital. Mr. Silverstein is obligated to transfer any shares issued under the options to Associates and Caduceus.

8. The stock option reported on this Form 4 was immediately exercisable as of the grant date for all shares of Common Stock that may be purchased under such option.

	/s/ Samuel D. Isaly	08/25/2008
	** 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 4 (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.