The blocking technology, intended to keep smut from children, does not violate the First Amendment even though it shuts off some legitimate, informational Web sites, the court held.

The 6-3 ruling reinstates a law that told libraries to install filters or surrender federal money. Four justices said the law was constitutional, and two others said it was allowable as long as libraries disable the filters for patrons who ask. The court described pornography in libraries as a serious problem.

"To the extent that libraries wish to offer unfiltered access, they are free to do so without federal assistance," the main ruling said.

Judith Krug, with the American Library Association, predicted that many libraries would consider turning down the money rather than installing filters. "We can't govern ourselves effectively if we don't have information available. It's not up to the filtering companies to decide," she said.

In Hawai'i, where the state library system oversees about 600 public access computers, officials need to analyze the decision before buying a filter, said Susan Nakata, head of the library development services section. Once that is done, the electronics staff will research the best filter, which could take about a month, she said.

"We will look for a filter that will comply with the ruling but will also enable us to disable it at the individual branch-level for patrons," Nakata said.

The filter would be placed on the system's main server in Salt Lake, she said.

Adult patrons will be able to ask librarians for full access to the Internet, but they still will not be allowed to access web sites that feature pornography, she said.

"We don't want to affect anyone else's enjoyment of the library, so we enforce those house rules," she said.

She did not know yet how much the filter would cost.

The court ruling today was victory for Congress, which has struggled to find ways to shield children from pornographic Internet sites. Congress has passed three laws since 1996; the first was struck down by the Supreme Court and the second was blocked by the court from taking effect.

The first two laws dealt with regulations on Web site operators. The latest approach, in the 2000 law, mandated that public libraries put blocking technology on computers as a condition for receiving federal money. Libraries have received about $1 billion since 1999 in technologies subsidies, including tax money and telecommunications industry fees.