The case has been closely watched by entertainment industry figures and privacy advocates as a key test for how easily file-swappers can be found and identified by copyright holders.
An earlier court decision that also said Verizon would have tohad been temporarily put on hold. Thursday's decision will force the telephone company to give up its subscriber's name in 14 days, unless an appeals court steps in to block the decision pending further review.
"Verizon cannot demonstrate that it has a substantial likelihood of prevailing...and has not shown that it will be irreparably harmed if a stay pending appeal is not granted," Judge John Bates wrote in his decision.
The decision closes a second round of fighting in district court over the Recording Industry Association of America's (RIAA's) attempt to subpoena Verizon for information about a subscriber accused of offering music files for download using Kazaa.
Bates earlier ruled that the RIAA was able to subpoena the subscriber's information under existing copyright law, even without an open legal case. Verizon had contended that the trade group needed to file suit against the anonymous subscriber for a subpoena to be valid.
Thursday's ruling stems from a new round of sparring, following a second RIAA subpoena issued in February. Verizon had asked that theon constitutional grounds and that the original decision be put on hold until an appeals court could rule on the issue.
Bates denied both motions. The subpoena laws that the RIAA is using do not violate constitutional separations of power, Bates said, nor do the laws violate computer users' free speech rights.
The RIAA, whose position had been supported by the U.S. Department of Justice in recent days, welcomed Bates' ruling.
"Today's decision makes clear that these individuals cannot rely on their (Internet service providers) to shield them from accountability," Cary Sherman, president of the RIAA, said in a statement. "If users of pirate peer-to-peer sites don't want to be identified, they should not break the law by illegally distributing music."
Verizon said it would immediately appeal the ruling.
"Today's ruling goes far beyond the interests of large copyright monopolists--such as the RIAA--in enforcing its copyrights," Verizon Senior Vice President John Thorne said in a statement. "This decision exposes anyone who uses the Internet to potential predators, scam artists and crooks, including identity thieves and stalkers. We will continue to use every legal means available to protect our subscribers' privacy and will immediately seek a stay from the U.S. Court of Appeals."