The decision by the court was in a case brought against biotechnology firm Amgen Inc over what kind of evidence must be presented before companies may be the subject of class-action lawsuits.
By a 6-3 vote, the court allowed the class action lawsuit against Amgen to go forward in which the Connecticut Retirement Plans and Trust Funds accuses the company of fraudulently inflating its stock price.
Justice Ruth Bader Ginsburg wrote that the initial stages of a class action are not designed to litigate the merits. Rather, she wrote, they are to select the best method to litigate fairly and efficiently.
Amgen "would have us put the cart before the horse," Ginsburg wrote.
The ruling featured an unusual voting lineup, as the court's four most liberal justices were joined by two conservatives, Chief Justice John Roberts and Justice Samuel Alito. Three conservatives dissented.
The Amgen activity in question took place between April 2004 and May 2007, when according to the shareholders in the suit the company exaggerated the safety of its anti-anemia drugs Aranesp and Epogen.
Shareholders sought class certification against Thousand Oaks, California-based Amgen on the "fraud on the market" theory endorsed by the Supreme Court in a 1988 case.
This assumes that the price of a stock in an efficient market reflects all public information, and that a purchaser is presumed to have relied on the truthfulness of that information.
Amgen said the shareholders should have been forced at the class certification stage rather than at trial to prove that its statements materially inflated the company's stock price, but a federal appeals court in Pasadena, California, let the class action proceed.
Amgen was supported in its appeals by various business groups, including the U.S. Chamber of Commerce, as well as former commissioners and officials at the U.S. Securities and Exchange Commission.
Most recently, in June 2011 the court decertified a class of as many as 1.5 million Wal-Mart Stores Inc female workers who accused the world's largest retailer of bias in pay and promotions, saying they raised too many different claims.
The case is Amgen Inc et al v. Connecticut Retirement Plans and Trust Funds, U.S. Supreme Court, No. 11-1085.
(Reporting by Lawrence Hurley and David Ingram; Editing by Howard Goller, Gerald E. McCormick and Andrew Hay)