The law banned sexual images purporting to be of children, but produced without the use of children. Such images could include actors who appear to be children, or may be completely computer generated without the use of real people.
The Child Pornography Prevention Act of 1996 "does more than prohibit pandering (of child pornography)," Justice Anthony Kennedy said in Tuesday's majority opinion. "It prohibits possession of material described, or pandered, as child pornography by someone earlier in the distribution chain. The provision prohibits a sexually explicit film containing no youthful actors, just because it is placed in a box suggesting a prohibited movie. Possession is a crime even when the possessor knows the movie was mislabeled. The First Amendment requires a more precise restriction. For this reason, (a key portion of the law) is substantially overbroad and in violation of the First Amendment."
Three justices dissented to part of the majority opinion.
In her dissent, Justice Sandra Day O'Connor said computer-generated images of child pornography may become so sophisticated that it will be hard to tell "virtual" porn from the real thing.
O'Connor said she would have struck down the part of the law that bans material that "'conveys the impression' that it contains actual-child pornography, but uphold the ban on pornographic depictions that 'appear to be' of minors so long as it is not applied to youthful-adult pornography."
Chief Justice William Rehnquist and Justice Antonin Scalia also issued partial dissents.
Justice Clarence Thomas joined in the majority's judgment, but wrote a separate opinion.
The case, which brought Tuesday's ruling, comes from San Francisco and deals in part with computer-generated images that may, or may not, involve real people.
Originally, the law banned "visual depictions of real children engaged in sexually explicit conduct," the Justice Department told the Supreme Court in a brief.
In the 1996 Child Pornography Prevention Act, however, Congress amended the law to include images of a child who "appears to be" engaging in sexual conduct, or images that "convey the impression" of child sex, regardless of whether real children were involved in the production.
The act bans the knowing possession, receipt, reproduction, sale, transmission or shipment of child pornography.
The 1996 act also listed 13 congressional "findings" to underpin the law, including these: That child pornography is used to seduce other children into sexual activity; that it is used to "whet the appetites" of pedophiles; and that advancing technology might make it impossible to distinguish between photographic and computer-generated images of children having sex.
After President Clinton signed the act into law, the Free Speech Coalition -- a trade association dealing in adult-oriented materials -- and other plaintiffs in Northern California challenged it in court.
A federal judge ruled that the law was constitutional in that it was not overbroad or too vague.
A federal appeals court panel reversed the judge, ruling that the "phrases 'appears to be' a minor,' and 'convey(s) the impression' that the depiction portrays a minor, are vague and overbroad" in the law "and thus do not meet the requirements of the First Amendment."
The Justice Department then asked the Supreme Court to intervene.
The justices heard argument in the case Oct. 30. Tuesday's ruling affirms the lower-court decision, striking down the federal law.
