File swapping is rising again as US ISPs turn their back on the Recording Industry Association of America (RIAA).
Market analyst the NPD Group said its MusicLab survey and MusicWatch Digital service showed that file swapping rose by one million users during November.
This increase follows declines from April 2003 to November 2003, after the RIAA began filing hundreds of lawsuits against individuals.
Industry experts suggested that one reason for the revival is that people feel safer as ISPs ignore the RIAA's pleas to voluntarily hand over the names of suspected file sharers.
A US court ruled in December that serving subpoenas forcing ISPs to disclose information on individuals was illegal.
NPD Group also said that November and December traditionally represent the high season for music sites as students have more time to download files.
A third factor could be the number of new legal music sites launched, which may have spurred on what NPD Group refers to as "legal-to-P2P site crossover".
The analyst explained that consumers were exploring legal digital music but could have drifted into the P2P realm to compare song file availability.
"We are just seeing the first increase in these numbers," said Russ Crupnick, vice president of NPD Group, in a statement.
"We will continue to monitor whether it is a temporary seasonal blip, or a trend which suggests that the industry should be more aggressive in capping the use of illegal methods to acquire digital music."
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