Exhibit 5.1

    

                                 May 19, 1999     



The Board of Directors
CAIS Internet, Inc.
1255 22nd Street, N.W.
Fourth Floor
Washington, D.C. 20037

     Re:  Registration Statement on Form S-1

Gentlemen:
    
     We have acted as counsel to CAIS Internet, Inc., a Delaware corporation
(the "Company"), with respect to the Company's Registration Statement on 
Form S-1 (the "Registration Statement"), filed with the Securities and Exchange
Commission, in connection with the registration under the Securities Act of
1933, as amended (the "Securities Act"), of up to 6,900,000 shares of the
Company's common stock, par value $.01 per share ("Common Shares"). This opinion
letter is furnished to you at your request to enable you to fulfill the
requirements of Item 601(b)(5) of Regulation S-K, 17 C.F.R. (S) 229.601(b)(5),
in connection with the Registration Statement.     

     As counsel to the Company, we have examined the Company's Certificate of
Incorporation, as amended (the "Certificates"), and such Company records,
certificates and other documents and relevant statutes, regulations, published
rulings and such questions of law as we considered necessary or appropriate for
the purpose of this opinion.

     In our examination, we have assumed the authenticity of original documents,
the accuracy of copies and the genuineness of signatures.  We have relied upon
the representations and statements of officers and other representatives of the
Company with respect to the factual determinations underlying the legal
conclusions set forth herein.  We have not attempted to verify independently
such representations and statements.
    
     This opinion letter is based as to matters of law solely on the corporate
law of the State of Delaware as currently in effect, and we express no opinion
herein as to any other laws, statutes, ordinances, rules or regulations.     

     Based upon, subject to and limited by the foregoing and the other
qualifications herein, we are of the opinion that, when the Registration
Statement has become effective under the Securities Act, upon issuance and
delivery of such Common Shares against payment of valid consideration 

 
The Board of Directors
February 22, 1999
Page 2

therefor as contemplated by the Registration Statement, such Common Shares will
be validly issued, fully paid and nonassessable.

     We hereby consent to the filing of this opinion as an exhibit to the
Registration Statement and to all references to our firm in the Registration
Statement.  In giving this consent, we do not admit that we come within the
category of persons whose consent is required under Section 7 of the Securities
Act or the rules promulgated thereunder.
    
     This opinion is rendered solely in connection with the transactions
described above. The opinions expressed in this letter are limited to the
matters expressly set forth herein, and no other opinions should be inferred
beyond the matters expressly stated herein.    


                               Very truly yours,
    

                               /s/ SWIDLER BERLIN SHEREFF FRIEDMAN, LLP     

                               SWIDLER BERLIN SHEREFF FRIEDMAN, LLP