The court, in a 7-2 decision, upheld a law aimed at cracking down on the flourishing online exchange of illicit images of children.

Joan Bertin, executive director of the National Coalition Against Censorship, said Justice Antonin Scalia's narrow reading of the law in his majority opinion should result in "considerably less damage than it might otherwise have done." But Bertin said aggressive prosecutors still could try to punish people for innocent activity and put them "through a terrible ordeal."

The ruling upheld part of a 2003 law that also prohibits possession of child pornography. It replaced an earlier law the court had struck down as unconstitutional.

The new law sets a five-year mandatory prison term for promoting, or pandering, child pornography. It does not require that someone actually possesses child pornography.

Opponents have said the law could apply to movies like "Traffic" or "Titanic" that depict adolescent sex or the marketing of other material that may not be pornography.

Scalia, in his opinion for the court, said the law takes a reasonable approach to the issue by applying it to situations where the purveyor of the material believes or wants a listener to believe that he has actual child pornography.

First Amendment protections do not apply to "offers to provide or requests to obtain child pornography," Scalia said.

Likewise, he said, the law does not cover "the sorts of sex scenes found in R-rated movies."

Justice David Souter, joined by Justice Ruth Bader Ginsburg, dissented. Souter said promotion of images that are not real children engaging in pornography still could be the basis for prosecution under the law. Possession of those images, on the other hand, may not be prosecuted, he said.

"I believe that maintaining the First Amendment protection of expression we have previously held to cover fake child pornography requires a limit to the law's criminalization of pandering proposals," Souter said.

Scalia said the law would not apply to a situation in which both sender and recipient were talking about virtual images, not real pictures.

Jay Sekulow, a conservative public interest lawyer who filed a brief on behalf of members of Congress in favor of the law, said the decision reflects the importance of trying to cut down on child pornography on the Internet.

"The court understood, perhaps for the first time, how difficult and troubling the proliferation of online pornography is," said Sekulow, of the American Center for Law and Justice.