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                                                                     EXHIBIT 5.1




                                                April 26, 2001


CSC Holdings, Inc.,
  1111 Stewart Avenue,
    Bethpage, New York 11714.

Ladies and Gentlemen:

            In connection with the registration under the Securities Act of 1933
(the "Act") of $1,000,000,000 principal amount of 7 5/8% Series B Senior Notes
due 2011 (the "Securities") of CSC Holdings, Inc., a Delaware corporation (the
"Company"), we, as your counsel, have examined such corporate records,
certificates and other documents, and such questions of law, as we have
considered necessary or appropriate for the purposes of this opinion.
            Upon the basis of such examination, we advise you that, in our
opinion, when the Registration Statement has become effective under the Act and
the Securities have been duly executed and authenticated in accordance with the
Indenture relating to the Securities and issued and delivered as contemplated in
the Registration Statement, the Securities will constitute valid and legally
binding
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CSC Holdings, Inc.                                             -2-



obligations of the Company, subject to bankruptcy, insolvency, fraudulent
transfer, reorganization, moratorium and similar laws of general applicability
relating to or affecting creditors' rights and to general equity principles.

            The foregoing opinion is limited to the Federal laws of the United
States, the laws of the State of New York and the General Corporation Law of the
State of Delaware, and we are expressing no opinion as to the effect of the laws
of any other jurisdiction.

            We have relied as to certain matters on information obtained from
public officials, officers of the Company and other sources believed by us to be
responsible, and we have assumed that the Indenture has been duly authorized,
executed and delivered by the Trustee thereunder, an assumption which we have
not independently verified.

            We hereby consent to the filing of this opinion as an exhibit to the
Registration Statement and to the reference to us under the heading "Validity of
the New Notes" in the Prospectus. In giving such consent, we do not
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CSC Holdings, Inc.                                             -3-


thereby admit that we are in the category of persons whose consent is required
under Section 7 of the Act.
                                                Very truly yours,

                                                SULLIVAN & CROMWELL