The survey released on Friday by the NPD Group showed music downloading using so-called peer-to-peer (P2P) services showed an increase in October and remained up in November.
This appears to be a reversal after declines linked to a lawsuit campaign last year, NPD analysts said.
"The music industry was heartened to see that their campaign to reduce or eliminate file sharing on P2P networks appeared to be working," said Russ Crupnick, vice president of NPD Group.
"Recent information from complementary data services at NPD, however, noted an increase in the number of households and individual consumers using P2P services to download digital music files."
NPD's data showed the number of households downloading digital music files was up 14% in November 2003 compared with September.
Increase after months of decline
The upturn came after six straight months of declines in digital file acquisition, since April 2003 when the Recording Industry Association of America began a campaign threatening individual file sharers with legal action resulting in fines.
The number remained below the peak of early last year. The survey indicated that 20 million individuals downloaded music from P2P services in May 2003, 18 million in July and 11 million in September. In November that figure was 12 million, which NPD called a "statistically significant increase."
Crupnick said some of the impact of the legal campaign may have faded, and indicated that the end of the year normally sees an increase in music purchases, which could spill over to downloading.
He also said the advent of legal music downloading services may have the unintended effect of sparking interest in file-sharing.
"It's important to keep in mind that file sharing is occurring less frequently than before the RIAA began its legal efforts to stem the tide of P2P file sharing," Crupnick said.
"We're just seeing the first increase in these numbers. NPD will continue to monitor whether it's a temporary seasonal blip, or a trend that suggests that the industry should be more aggressive in capping the use of illegal methods to acquire digital music."