The recording industry can still bring civil lawsuits against people who download music illegally, but Friday's court ruling will make that more expensive and time-consuming.
A federal appeals court said Internet providers, such as Verizon, EarthLink and America Online, do not have to turn over the names of their customers when music companies serve them with a subpoena.
The industry had been relying on such subpoenas to find out the names of those they suspect of online music piracy. More than 300 lawsuits have already been filed against individuals, many of whom have settled for thousands of dollars.
Legal experts say the recording industry can still bring civil lawsuits against individuals, even without knowing their identity, by filing what is known as a "John Doe" lawsuit.
Under those guidelines, the industry's lawyers could ask a judge for permission to send out subpoenas, legal experts said.
"That's a time-consuming and fairly expensive process," said Daniel Ballard, a Sacramento-based intellectual property attorney who represents an unidentified woman who had sought to block Verizon from releasing her identity to the recording industry.