Exhibit 5.1

                                McGuireWoods LLP
                         9 West 57th Street, Suite 1620
                          New York, New York 10019-2602

October 22, 2002



Peter Klein
Chief Executive Officer
Diomed Holdings, Inc.
1 Dundee Park
Andover, MA 01810

Dear Mr. Klein,

         We are acting as counsel to Diomed Holdings, Inc., a Delaware
corporation (the "Company"), in connection with the filing with the Securities
and Exchange Commission (the "Commission"), under the Securities Act of 1933, as
amended (the "Act"), the Registration Statement on Form SB-2 dated June 14,
2002, as amended by Pre-Effective Amendment No. 1 thereto, dated August 7, 2002,
and as further amended by Pre-Effective Amendment No. 2 thereto, dated
September 10, 2002 and by Pre-Effective Amendment No. 3 thereto, dated
October 22, 2002 (the "Registration Statement"), relating to the offer
and sale pursuant to the Registration Statement, by the Selling Stockholders
identified in the Registration Statement, of up to 20,919,470 shares of Common
Stock, par value $0.001 per share of the Company, issuable in connection with
the following:

     (i)    5,000,000 shares of Common Stock to be offered by selling
            stockholders;

     (ii)   15,461,749 shares of Common Stock issuable upon conversion of the
            Company's Convertible Preferred Stock;

     (iii)  121,924 shares of Common Stock issuable upon conversion of
            Convertible Preferred Stock issuable upon the exercise of Company
            Warrants for such Convertible Preferred Stock; and

     (iv)   335,797 shares of Common Stock issuable upon conversion of
            Convertible Preferred Stock issuable upon exercise of Company
            options for such Convertible Preferred Stock.

         This opinion letter is furnished to you for filing with the Commission
pursuant to Item 601 of Regulation S-B promulgated under the Act. Capitalized
terms not defined herein shall have the meanings ascribed thereto in the
Registration Statement.

         In reaching the opinion stated in this letter, we have reviewed
originals or copies of the Registration Statement, the Certificate of
Incorporation and the By-laws of the Company and such other documents as we have
considered relevant. We have assumed that (i) all information contained in all
documents reviewed by us is correct, (ii) all signatures on all documents
reviewed by us are genuine, (iii) all documents submitted to us as originals are
true and complete, (iv) all documents submitted to us as copies are true and
complete copies of the originals thereof, (v) each natural person signing any
document



October 22, 2002
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reviewed by us had the legal capacity to do so and (vi) each natural person
signing in a representative capacity any document reviewed by us had authority
to sign in such capacity.

         Based upon the foregoing, it is our opinion that the shares of Common
Stock to be (i) offered by selling stockholders, (ii) issuable upon conversion
of the Company's Convertible Preferred Stock, (iii) issuable upon conversion of
Convertible Preferred Stock issuable upon the exercise of Company Warrants for
such Convertible Preferred Stock, and (iv) issuable upon conversion of
Convertible Preferred Stock issuable upon exercise of Company options for such
Convertible Preferred Stock, when issued in the manner described in the
Registration Statement, will be authorized, validly issued, fully paid and
nonassessable.

         The opinion expressed above is limited to matters governed by the laws
of the State of Delaware. We express no opinion herein about the effect of
federal or state securities laws or the laws of any other jurisdiction.

         We hereby consent to the use of this opinion letter as Exhibit 5.1 to
the Registration Statement and to the use of our name under the heading "Legal
Matters" therein. In giving this consent, we do not admit that we are in the
category of persons whose consent is required under Section 7 of the Act or the
rules and regulations of the Commission promulgated thereunder.

         This opinion letter speaks as of the date hereof. We disclaim any duty
to advise you regarding any change subsequent to the date hereof in, or to
otherwise communicate with you with respect to, the matters addressed herein.

                                            Very truly yours,



                                            /s/ McGuireWoods, LLP