In a 1942 decision upholding a New York City ban on handbills, the U.S. Supreme Court unanimously declared that "the Constitution imposes no...restraint on government as respects purely commercial advertising." The four-page opinion cited no authority and offered no justification for this assertion.

Although it has been watered down over the years, the commercial speech doctrine has been with us ever since, inviting the suppression of controversial messages tainted by a profit motive. Now the Supreme Court has agreed to hear a case that will give it an opportunity to close this gaping loophole in the First Amendment once and for all.

The case challenges restrictions on tobacco advertising imposed by the Massachusetts attorney general. The regulations ban outdoor advertising, including signs inside stores that are visible from the street, within 1,000 feet of a playground or school. They also require that indoor advertising within such a zone be placed at least five feet from the floor.

The regulations, which ostensibly are aimed at protecting children, in practice bar outdoor ads for cigars, cigarettes, and smokeless tobacco from about 90 percent of the Boston, Worcester, and Springfield metropolitan areas. The U.S. Court of Appeals for the 1st Circuit nevertheless concluded that the rules passed the constitutional test for restrictions on commercial speech.

That test, set forth by the Supreme Court in 1980, says the regulation of truthful, nonmisleading commercial speech should be upheld only if it directly advances a substantial government interest and is no more extensive than necessary. While these requirements sound stringent, they have little or no force when applied by a credulous, deferential court, as the Massachusetts case illustrates.

During the last decade, the Supreme Court itself has become increasingly skeptical about arguments for restricting commercial speech. In 1995, for instance, the Court unanimously ruled that a federal regulation barring brewers from listing alcohol content on their beer labels violated the First Amendment. The following year, it unanimously overturned a Rhode Island law against advertising liquor prices.

The question now is why the Court should continue to draw a line between commercial and noncommercial speech, a distinction that has never been clearly stated, let alone justified. What makes the Playboy Channel or a Stephen King novel more worthy of protection than a Snickers commercial? All three are intended to make money, and all three (arguably) have artistic content.

If concerns about underage smoking can justify bans on cigarette ads, it is not obvious why they do not also justify restricting noncommercial messages that might encourage kids to smoke. What about scenes in books or movies that portray smoking as cool or pleasurable?

To take a closer example, suppose an eccentric millionaire who was not in the cigarette business wanted to set up a billboard across the street from a Boston high school showing sexy, fashionable people having fun while smoking cigarettes. More plausibly, suppose a political group tried to sponsor a billboard urging passers-by to smoke as an act of rebellion against paternalistic government.

On their face, the Massachusetts regulations, which apply to advertising "the purpose or effect of which is to promote the use or sale of the product," would forbid such billboards. But since these messages are noncommercial, they presumably would be protected by the First Amendment. Go figure.

In addition to propagating mysterious distinctions, the second-class status of so-called commercial speech threatens to undermine protection of political, scientific, and artistic expression. Consider "advertorials," in which companies take positions on policy issues that affect their bottom lines. Are these messages commercial or noncommercial?

Manufacturers of drugs and vitamin supplements use articles from medical journals to promote their products. Does that mean the articles can be regulated as advertising?

Product placement and product tie-ins can make movies and TV shows resemble ads. At the extreme, some programs and channels are devoted entirely to showing off merchandise. Is there a clear distinction between a commercial that entertains and a movie or TV show that sells stuff?

As federal appeals court judge Alex Kozinski and attorney Stuart Banner noted in a 1990 Virginia Law Review article, the commercial speech doctrine "gives government a powerful weapon to suppress or control speech by classifying it as merely commercial. If you think carefully enough, you can find a commercial aspect to almost any first amendment case." But if you think carefully enough, you'll realize there's no point to looking.