The Supreme Court handed tobacco companies a major and well-deserved setback on Monday, saying smokers may proceed with lawsuits arguing that they were deceived by the marketing of “light” cigarettes.

The 5-to-4 ruling  with Justice Anthony Kennedy casting his swing vote with the court’s four most liberal members  could do a great deal to rein in deceptive advertising by cigarette manufacturers. It was a welcome departure for a court that has been far too deferential to business. We hope it signals that the justices are moving toward a more balanced approach to business cases.

The case was brought by Maine residents who have smoked light cigarettes made by Philip Morris, whose parent company is Altria, for more than 15 years. They argue that Philip Morris’s contention that these cigarettes deliver less tar and nicotine to smokers than regular brands was untrue.

Even though each light cigarette has less tar and nicotine than a regular cigarette, the plaintiffs say that Philip Morris knew that smokers unconsciously smoke them differently, taking larger puffs, for instance, or holding the smoke longer in their lungs to make up for the difference in the cigarettes’ strength. Because that allows them to extract the same amount of tar and nicotine as they would from regular cigarettes, the smokers argue that the company’s marketing violates the Maine Unfair Trade Practices Act’s prohibition on deceptive business practices.