Freeserve said that by December 31, like-for-like active 40-day registered accounts increased 13% on the same period in 2000 to 2.3m.
The company said it had netted more than 250,000 30-day Freeserve accounts in 2001 and claims to have almost 50% more customers than the 1.5m its closest rival AOL claims.
Freeserve's unmetered subscriber base continued to grow during the fourth quarter, with almost one third of all customers taking a Freeserve paid-for package.
Freeserve said the gains for the full year were almost entirely attributable to the fourth quarter and its renewed marketing activity.
The multimillion-pound campaign, created by M&C Saatchi, used the theme "be freeserve". One ad showed naturists running around in the nude, while a second execution featured a bunch of hippies.
The ads aimed to highlight Freeserve's pledge to offer access to the whole internet rather than access to a walled garden in the same way as rival ISPs.
The ads also drew attention to the ISP's £12.99-a-month unmetered internet access package, Freeserve AnyTime.
The ads were carried on TV, radio and press, as well as direct marketing and consumer offers in Dixons Group stores.
Meanwhile, Freeserve has filed a complaint with the US Securities and Exchange Commission over AOL Time Warner's claim in a financial press release dated January 6 that "in the UK, AOL is the leading ISP with more than 1.5m members".
"Freeserve believes the statement, made in the context of AOL Time Warner's preliminary results for 2001, to be misleading to investors to a material degree and without significant verification presents a distorted picture of AOL's position in the UK," Freeserve said in a statement.
Freeserve CEO John Pluthero said: "Given that AOL has been sanctioned by the ASA about misleading claims made in its advertising and even by the SEC before for improper accounting practices, perhaps it should come as no surprise that it is once again found peddling wild and extravagant claims."
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