The CDA was an attempt to protect minors from explicit material on the Internet by criminalizing the "knowing" transmission of "obscene or indecent" messages to any recipient under 18; and also the knowing sending to a person under 18 of anything "that, in context, depicts or describes, in terms patently offensive as measured by contemporary community standards, sexual or excretory activities or organs."
In Ginsberg v. New York, the Supreme Court had upheld a New York statute that barred selling obscene material to minors under 17; in Reno v. ACLU, the government argued that the CDA was a similar statute applying instead to the Internet. The Court, however, noted that the CDA was much broader than the New York statute, which allowed parents to consent to purchases; applied only to commercial transactions; and as part of the definition of "obscene", said the material must be "utterly without redeeming social importance for minors". (The latter echoed a requirement in the decision overturning a U.S. government ban on the book Ulysses by James Joyce 80 years previous.)
In F.C.C. v. Pacifica Foundation, the Supreme Court had upheld the possibility of the FCC delivering administrative sanctions to a radio station for broadcasting George Carlin's monologue titled "Filthy Words". In Reno v. ACLU, though, the Supreme Court held that this was not case law justifying the CDA, as the FCC's sanctions were not criminal punishments; and TV and radio broadcasts, "as a matter of history, had 'received the most limited First Amendment protection' … in large part because warnings could not adequately protect the listener from unexpected program content", as opposed to Internet users, who must take "a series of affirmative steps" to access explicit material.
Finally, in Renton v. Playtime Theaters, Inc., the Supreme Court had upheld a zoning ordinance that kept adult movie theaters out of residential neighborhoods. The government argued that the CDA was an attempt to institute "a sort of 'cyberzoning' on the Internet". In Reno v. ACLU, however, the Court ruled that the "time, place, and manner regulation" that Renton had enacted was not similar to the CDA, which was "a content-based blanket restriction on speech".
The two dissented in part, writing they would have invalidated a narrower portion of the two CDA provisions under review.