Exhibit 5

                               Hale and Dorr LLP
                               Counselors at Law
                  60 State Street, Boston, Massachusetts 02109
                        617-526-6000 * FAX 617-526-5000

                               November 12, 1999


Sycamore Networks, Inc.
10 Elizabeth Drive
Chelmsford, MA 01824

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


Ladies and Gentlemen:

     This opinion is furnished to you in connection with a Registration
Statement on Form S-8 (the "Registration Statement") filed with the Securities
and Exchange Commission (the "Commission") under the Securities Act of 1933, as
amended (the "Securities Act"), for the registration of 19,382,553 shares of
Common Stock, $.001 par value per share (the "Shares"), of Sycamore Networks,
Inc., a Delaware corporation (the "Company"), issuable under the Company's 1999
Employee Stock Purchase Plan, 1999 Non-Employee Directors Option Plan, 1999
Stock Incentive Plan and 1998 Stock Incentive Plan (the "Plans"). Certain of the
Shares are being offered and sold by the Company pursuant to the Plans (the
"Primary Shares") and certain of the Shares are being registered for resale by
certain stockholders of the Company and are being offered pursuant to the resale
prospectus included in the Registration Statement (the "Reoffered Shares").

     We have examined the Certificate of Incorporation and By-Laws of the
Company, each as amended and restated to date, and originals, or copies
certified to our satisfaction, of all pertinent records of the meetings of the
directors and stockholders of the Company, the Registration Statement and such
other documents relating to the Company as we have deemed material for the
purposes of this opinion.

     In examination of the foregoing documents, we have assumed the genuineness
of all signatures, the authenticity of all documents submitted to us as
originals, the conformity to original documents of all documents submitted to us
as copies, the authenticity of the originals of such latter documents and the
legal competence of all signatories to such documents.

     We express no opinion herein as to the laws of any state or jurisdiction
other than the state laws of the Commonwealth of Massachusetts, the Delaware
General Corporation Law statute and the federal laws of the United States of
America.

     Based upon and subject to the foregoing, we are of the opinion that (i) the
Shares have been duly authorized for issuance and (ii) the Reoffered Shares and,
when issued and paid for in accordance with the terms of the Plans, the Primary
Shares, will be validly issued, fully paid and nonassessable.

     It is understood that this opinion is to be used only in connection with
the offer and sale of the Shares while the Registration Statement is in effect.