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                                                                     Exhibit 5.1



                                             January 25, 1999


PAREXEL International Corporation
195 West Street
Waltham, MA 02154


         Re:  Registration Statement on Form S-8 Relating to the 1998
              Non-Qualified, Non-Officer Stock Option Plan (the "Plan") of
              PAREXEL International Corporation (the "Company")



Dear Sir or Madam:

         Reference is made to the above-captioned Registration Statement on Form
S-8 (the "Registration Statement") to be filed by the Company on or about
January 25, 1999 with the Securities and Exchange Commission under the
Securities Act of 1933, as amended, relating to an aggregate of 1,000,000 shares
of Common Stock, $.01 par value per share, of the Company issuable pursuant to
the Plan (the "Shares").

         We have examined, are familiar with, and have relied as to factual
matters solely upon, a copy of the Plan, the Amended and Restated Articles of
Organization, as amended, and the Amended and Restated By-laws of the Company,
the minute books and stock records of the Company and originals of such other
documents, certificates and proceedings as we have deemed necessary for the
purpose of rendering this opinion.

         Based on the foregoing, we are of the opinion that the Shares have been
duly authorized and, when issued and paid for in accordance with the terms of
the Plan, will be validly issued, fully paid and non-assessable.

         We consent to the use of this opinion as Exhibit 5.1 to the
Registration Statement, and further consent to the use of our name wherever
appearing in the Registration Statement and any amendments thereto.



                                           Very truly yours,


                                           /s/ Testa, Hurwitz & Thibeault, LLP
                                           -----------------------------------
                                           TESTA, HURWITZ & THIBEAULT, LLP