ONE NEEDN'T BE a bluenose to worry about children's access to pornography on the Internet. Children are surfing the Web and exchanging e-mails at younger and younger ages; in 2001, more than two-thirds of children ages 9 to 17 were Internet users. Meantime, the Internet has become a cyber-bazaar of pornography, with sexually explicit images available at the click of a mouse -- either intentionally or because many porn sites are designed to lure the unwary Web user. According to a report commissioned by Congress, some 70 million individuals visit pornographic Web sites each week; about 11 million of them are younger than 18. And this is not Playboy magazine; many sites feature pictures and videos far more graphic than any centerfold.

It's easier, though, to diagnose the problem than to come up with an effective solution -- particularly one that will pass constitutional muster, as the Supreme Court has demonstrated three times now. Seven years ago, the court properly rejected Congress's ham-handed first effort. This week, for the second time, the justices blocked enforcement of a more carefully crafted but still problematic law, the Child Online Protection Act. The law makes it a crime, punishable by a $50,000 fine and six months in prison, to post online, for "commercial purposes," sexual material that is "harmful to minors." Web sites can avoid those restrictions if they restrict access to minors by requiring use of a credit card or some other proof of adulthood.

The five-justice majority didn't find that the law was unconstitutional; rather, the justices sent the case back to a lower court to determine whether it imposed the "least restrictive alternative" on free speech. In particular, the justices pointed to the availability of filtering software to block minors' access to offensive Web sites. This software, the court said, infringes less on speech than the statute and might be more effective, because the law applies only to pornography posted from the United States. Much of the explicit material -- 40 percent, according to one expert -- comes from overseas, and more could move offshore if the law is upheld. And the software can block not only Web sites but indecent e-mail as well.

As the majority acknowledged, the software "is not a perfect solution." It has a fee, and it isn't completely effective: Some troublesome sites can slip through, while useful ones can be blocked. It depends on parental involvement in an age when children are often left unsupervised. We don't doubt the technological ingenuity of today's teenagers in overcoming whatever cyber-barriers their less Web-savvy parents seek to impose.

But we think the majority was right to send the case back for review. The law's criminal penalties on speech give us pause, as does the requirement that adults, even those with no children in their household, submit identifying information before being allowed to access sites that they're entitled to view. That's a more intrusive requirement than, say, showing a driver's license when buying liquor or cigarettes.

Meanwhile, in the six years since the bill was passed, filters have become more effective, and Congress has passed two other laws aimed at addressing the problem of Internet por- nography: a ban on misleading domain names and a law creating a safe domain, "dot kids." If the government can prove that the Child Online Protection Act remains, notwithstanding all that, the least restrictive way to shield children, the court's ruling gives it another chance to do so.