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                                                                     Exhibit 5.1


                      POSTERNAK, BLANKSTEIN & LUND, L.L.P.
                                ATTORNEYS AT LAW
                                   ----------
                             100 CHARLES RIVER PLAZA
                        BOSTON, MASSACHUSETTS 02114-2723


TEL  617-973-6100
FAX  617-367-2315
E-MAIL  pbl@pbl.com


                                                     April 16, 1998


       National Dentex Corporation
       526 Boston Post Road
       Wayland, Massachusetts 01778

         Re:      Registration Statement On Form S-8
                  ----------------------------------


       Ladies and Gentlemen:

              This opinion is rendered to you in connection with the preparation
       of the Registration Statement on Form S-8 (the "Registration Statement")
       under the Securities Act of 1933, as amended (the "Securities Act"), by
       National Dentex Corporation, a Massachusetts corporation (the "Company"),
       relating to an aggregate of 150,000 shares (the "Shares") of the
       Company's common stock, $.01 par value per share (the "Common Stock"),
       issuable upon the exercise of options granted under the Company's 1992
       Long Term Incentive Plan (the "Plan").

              We have acted as counsel to the Company in connection with the
       preparation of the Registration Statement. For purposes of this opinion,
       we have examined: the Articles of Organization and By-laws, each as
       amended and restated, of the Company; such records of the corporate
       proceedings of the Company as we have deemed material; the Registration
       Statement and all exhibits thereto; and such other documents as we have
       deemed necessary to enable us to render this opinion.

              We are attorneys admitted to practice in The Commonwealth of
       Massachusetts. We express no opinion concerning the laws of any
       jurisdiction other than the laws of the United States of America and The
       Commonwealth of Massachusetts.

              In rendering the opinion expressed herein, we assume that all
       steps necessary to comply with the registration requirements of the
       Securities Act and with applicable requirements of state law regulating
       the sale of securities will be duly taken.





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POSTERNAK, BLANKSTEIN & LUND, L.L.P.

National Dentex Corporation
April 16, 1998
Page 2




       Based upon and subject to the foregoing, and having regard for such legal
considerations as we have deemed relevant, it is our opinion that the Shares
have been duly authorized for issuance by all necessary corporate action and,
when issued and delivered against payment of the consideration therefor as set
forth in the Plan, will be validly issued, fully paid and nonassessable.

       This opinion is rendered solely for your benefit, and may not be relied
upon by any other party without our prior written consent. We hereby consent to
the filing of this opinion as an exhibit to the Registration Statement and to
the reference to our name in the Registration Statement.



                                        Very truly yours,


                                        POSTERNAK, BLANKSTEIN & LUND, L.L.P.



                                        By: /s/ Donald H. Siegel, P.C.
                                            ----------------------------------- 
                                            A Partner Thereof