Long-awaited Doom 3 leaked online
Copies of the game on file-sharing networks and newsgroups are being downloaded by thousands of people.
At one point on Sunday, more than 50,000 copies of the game, which weighs in at more than 1.5 gigabytes, were being downloaded.
Translated in lost sales, this amounts to $2.7m (£1.5m), if all these people instead bought the game.
"Despite the relatively low price of PC games, many gamers are still choosing to resort to piracy rather than pay for legitimate boxed copies," said Matt Pierce, publisher of the computer games magazine, PC Gamer.
"Whereas in the case of Doom 3, it almost certainly won't prevent the game being a massive seller, it will still cost both the publishers and developers of the game millions of dollars in lost revenue, an outcome that can only serve to harm future game development on the PC," he told BBC News Online.