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/A of NxStage
Medical, Inc. for the period ended June 30, 2011 (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)
    
    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):b)
    
    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.
 Chief Financial Officer
(principal financial and accounting officer)