EXHIBIT 31.2
 CERTIFICATION PURSUANT TO RULE 13A-14(A)/15D-14(A),
AS ADOPTED PURSUANT TO
SECTION 302 OF THE SARBANES-OXLEY ACT OF 2002
  1.
    
    I have reviewed this Quarterly Report on Form 10-Q of NxStage Medical, Inc. for the period ended September 30,
2010 (this report);2.
    
    Based on my knowledge, this report does not contain any
untrue statement of a material fact or omit to state a
material fact necessary to make the statements made, in
light of the circumstances under which such statements
were made, not misleading with respect to the period
covered by this report;3.
    
    Based on my knowledge, the financial statements, and
other financial information included in this report,
fairly present in all material respects the financial
condition, results of operations and cash flows of the
registrant as of, and for, the periods presented in this
report;4.
    
    The registrants other certifying officer and I are
responsible for establishing and maintaining disclosure
controls and procedures (as defined in Exchange Act
Rules 13a-15(e) and 15d-15(e)) and internal control over
financial reporting (as defined in Exchange Act Rules
13a-15(f) and 15d-15(f)) for the registrant and have:
 a)
    
    Designed such disclosure controls and procedures, or
caused such disclosure controls and procedures to be
designed under our supervision, to ensure that material
information relating to the registrant, including its
consolidated subsidiaries, is made known to us by others
within those entities, particularly during the period in
which this report is being prepared;b)
    
    Designed such internal control over financial reporting,
or caused such internal control over financial reporting
to be designed under our supervision, to provide
reasonable assurance regarding the reliability of
financial reporting and the preparation of financial
statements for external purposes in accordance with
generally accepted accounting principles;c)
    
    Evaluated the effectiveness of the registrants disclosure
controls and procedures and presented in this report our
conclusions about the effectiveness of the disclosure
controls and procedures, as of the end of the period
covered by this report based on such evaluation; andd)
    
    Disclosed in this report any change in the registrants
internal control over financial reporting that occurred
during the registrants most recent fiscal quarter (the
registrants fourth fiscal quarter in the case of an
annual report) that has materially affected, or is
reasonably likely to materially affect, the registrants
internal control over financial reporting; and
 5.
    
    The registrants other certifying officer and I have disclosed, based
on our most recent evaluation of internal control over financial
reporting, to the registrants auditors and the audit committee of the
registrants board of directors (or persons performing the equivalent
functions):
 a)
    
    All significant deficiencies and material weaknesses in
the design or operation of internal control over financial
reporting which are reasonably likely to adversely affect
the registrants ability to record, process, summarize and
report financial information; andb)
    
    Any fraud, whether or not material, that involves
management or other employees who have a significant role
in the registrants internal controls over financial
reporting.
 /s/ Robert S. BrownRobert S. BrownChief Financial Officer(principal financial and accounting officer)Date: November 4, 2010