Exhibit 5.1

The Board of Directors
Aetna Inc.
151 Farmington Avenue
Hartford, Connecticut 06156

Ladies and Gentlemen:

Aetna Inc., a Pennsylvania corporation (the “Company”) has filed with the Securities and Exchange Commission a Registration Statement on Form S-3 (File No. 333-200647) (the “Registration Statement”) for the purpose of registering under the Securities Act of 1933, as amended (the “Securities Act”), certain securities, including $1,000,000,000 aggregate principal amount of its 3.875% Senior Notes due 2047 (the “Securities”). The Securities are to be issued pursuant to the provisions of the Senior Indenture dated as of March 2, 2001 (the “Base Indenture”) between the Company and U.S. Bank National Association, successor-in-interest to State Street Bank and Trust Company, as trustee (the “Trustee”), as supplemented by a supplemental indenture to be dated as of August 10, 2017 (together with the Base Indenture, the “Indenture”) between the Company and the Trustee, and to be sold pursuant to the Pricing Agreement dated as of August 7, 2017 (the “Pricing Agreement”) among the Company and the several underwriters named in Schedule I thereto (the “Underwriters”).

We, as your counsel, have examined originals or copies of such documents, corporate records, certificates of public officials and other instruments as we have deemed necessary or advisable for the purpose of rendering this opinion.

In rendering the opinion expressed herein, we have, without independent inquiry or investigation, assumed that (i) all documents submitted to us as originals are authentic and complete, (ii) all documents submitted to us as copies conform to authentic, complete originals, (iii) all documents filed as exhibits to the Registration Statement that have not been executed will conform to the forms thereof reviewed by us, (iv) all signatures on all documents that we reviewed are genuine, (v) all natural persons executing documents had and have the legal capacity to do so, (vi) all statements in certificates of public officials and officers of the Company that we reviewed were and are accurate and (vii) all representations made by the Company as to matters of fact in the documents that we reviewed were and are accurate.

Based upon the foregoing, and subject to the additional assumptions and qualifications set forth below, we advise you that, in our opinion, when the Securities have been duly executed and authenticated in accordance with the provisions of the Indenture and delivered to and paid for by

the Underwriters pursuant to the Pricing Agreement, the Securities will constitute valid and binding obligations of the Company, enforceable in accordance with their terms, subject to applicable bankruptcy, insolvency and similar laws affecting creditors’ rights generally and the rights and remedies of creditors of insurance companies generally, concepts of reasonableness and equitable principles of general applicability, provided that we express no opinion as to the validity, legally binding effect or enforceability of any provision that permits holders to collect any portion of stated principal amount upon acceleration of the Securities to the extent determined to constitute unearned interest.

In connection with the opinion expressed above, we have assumed that (i) the Registration Statement became effective upon filing with the Commission and such effectiveness shall not have been terminated or rescinded; (ii) the Indenture and the Securities are valid, binding and enforceable agreements of each party thereto (other than as expressly covered above in respect of the Company); and (iii) there shall not have occurred any change in law affecting the validity or enforceability of the Securities. We have also assumed that the execution, delivery and performance by each party to each Document to which it is a party (a) are within its corporate powers, (b) do not contravene, or constitute a default under, the certificate of incorporation or bylaws or other constitutive documents of such party, (c) require no action by or in respect of, or filing with, any governmental body, agency or official and (d) do not contravene, or constitute a default under, any provision of applicable law or regulation or any judgment, injunction, order or decree or any agreement or other instrument binding upon such party, provided that we make no such assumption to the extent that we have specifically opined as to such matters with respect to the Company.

We are members of the Bar of the State of New York, and the foregoing opinion is limited to the laws of the State of New York and the federal laws of the United States of America. Insofar as the foregoing opinion involves matters governed by the laws of the Commonwealth of Pennsylvania, we have relied, without independent inquiry or investigation, on the opinion of even date herewith of Drinker Biddle & Reath LLP, special Pennsylvania counsel for the Company, to be filed as an exhibit to a report on Form 8-K to be filed by the Company on the date hereof.

We hereby consent to the filing of this opinion as an exhibit to a report on Form 8-K to be filed by the Company on the date hereof and its incorporation by reference into the Registration Statement and further consent to the reference to our name under the caption “Validity of the Notes” in the prospectus supplement which is a part of the Registration Statement. 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 Securities Act.