Invalidation of a law because it is thought to be too broad is “strong medicine” that is not to be “casually employed,” Justice Scalia wrote, citing earlier Supreme Court rulings and declaring that the United States Court of Appeals for the 11th Circuit had employed the strong medicine too casually in the case at hand.

Monday’s decision in United States v. Williams reinstated the conviction of Michael Williams of Florida, who was caught in a federal undercover operation in April 2004 and found guilty later of “pandering” child pornography, a charge defined in part as promoting or distributing real or “purported” material in a way that reflects the belief  or is intended to persuade another  that the material is indeed child pornography.

Mr. Williams was nabbed offering to trade nude pictures of his young daughter and other forms of child pornography in an Internet chat room. He did not actually have pictures of his daughter, but he did have 22 pornographic images of other children on his computer hard drive. He pleaded guilty to possessing that material, which has long been a crime, and was sentenced to five years in prison. That aspect of the case against him was not part of the Supreme Court argument.

Instead, lawyers on his behalf challenged the “pandering” charge, which also carries a five-year sentence, asserting that it was so “overly broad” as to violate the First Amendment guarantee of free speech. The 11th Circuit found that argument persuasive, reasoning that offering a copy of “Snow White” on false claims that it depicts minors engaging in sex could be construed as criminal behavior.

But Justice Scalia dismissed the 11th Circuit’s finding, its reliance on what he considered far-fetched hypothetical situations and the notion that the statute under review would cause all sorts of fact-finding problems. Judges and juries are routinely called upon to assess difficult issues of fact and intent involving charges like conspiracy, incitement and solicitation, he wrote.