Gone are the furtive visits to seedy theaters and the fear of being outed as some perverted purchaser of porn. Now, all you need to indulge anonymously in the "XXX" world is your trusty personal computer and a good connection to the Internet.

It's difficult to derive reliable figures from an industry that, despite flirtations with the mainstream, is made up of many small shops that prefer to keep a low profile. But the figures that exist paint a picture of a booming online field, fueled by the relatively low costs of setting up shop, fickle consumers in constant search of new thrills and the promise of quick profits.

The flood of sites competing for attention is fueling a torrent of X-rated spam, resulting in minors being exposed to adult content and annoying marketing ploys that spurred the recent approval in Congress of the first national effort to stem the flood of unwanted e-mail.

Attorney General John Ashcroft's office has launched dozens of investigations of adult content businesses and filed an obscenity case against Extreme Associates, a California porn firm that sold violent sex videos by mail and over the Internet.

It's an enormous business ... There's a lot of money to be made, said Sean Kaldor, an analyst with Nielsen/NetRatings, which estimated that 34 million visited porn sites in August -- about one in four Internet users in the United States. The average user is "looking at 121 pages, going back six times and spending an hour and seven minutes every month looking at adult-related material," Kaldor said.

All that browsing has caused the number of pornography Web pages to soar during the past six years, with over 1.3 million sites serving up about 260 million pages of erotic content, according to a study released in September by the Seattle, Washington-based Web-filtering company N2H2.

N2H2's database of porn sites, a company spokesman said, includes many low-budget, fly-by-night and sometimes unscrupulous operators hoping to rake in their share of a market that the National Research Council estimates to be in the $1 billion range annually.

The council, which advises Congress on technology, issued a report in 2002 that predicts the online porn industry will grow to a $5-$7 billion business within five years.

Kathee Brewer, technology editor of porn industry news site AVN Online, said the increase in adult Internet pages has spurred opposition from conservative groups and heightened government scrutiny. She said critics of porn sites are attempting to blur the lines between law-abiding adult content and banned obscene material.

People can be easily led, and the mere twist of a phrase -- like substituting 'obscenity' for 'pornography' -- can have a profound effect on basically good folk who want to do the right thing but don't know exactly how to go about it, Brewer wrote recently in an essay about conservative groups that support porn-filtering software.

Instead of government intervention, Brewer urged the industry to police itself by keeping minors away from explicit content and cutting down on spam e-mail. At the same time, she said, it should be acknowledged that porn has been one of the few profitable Internet businesses from the start, employing thousands of people and generating millions in revenues for site owners, Web hosting companies and computer-hardware firms.

Experts say the industry has been on the forefront of many innovations that have been adopted by mainstream sites, such as new payment systems, ad revenue models, chat and broadband.

One of the most interesting things is to watch how these sites pioneer new technologies, said Kaldor, the Nielsen/NetRatings analyst. Kaldor said the industry is showing signs of maturity.

Password services have sprung up, often charging an annual fee to deliver access to hundreds of small sites, which share the subscription revenues.

Large firms also have consolidated power by providing free content to smaller "affiliate" sites. The affiliates post the free content and then try to channel visitors to the large sites, which give the smaller sites a percentage of the fees paid by those who sign up.

Another way some adult Webmasters make money is by forwarding traffic to another porn site in return for a small per-consumer fee. In many cases, the consumer is sent to the other sites involuntarily, which is known in the industry as "mousetrapping." Surfers who try to close out a window after visiting an adult site are sent to another Web page automatically. This can repeat dozens of times, causing users to panic and restart their computers in order to escape, the National Research Council found.

A fourth trend is for adult sites to cater to niche audiences. There's a Web site for just about every kink, said Scott Fayner, who writes for LukeFord.com, a site that posts porn industry news and gossip. Experts say tech advances and the growing use of broadband will fuel even more growth in the industry.

All of which is prompting concerns about what impact the onslaught of porn might have on future generations raised on a steady stream of adult images. Some believe porn is creating unrealistic expectations among couples.

Like it or hate it, Internet porn is here to stay, Amsden said. And the key, said sex therapist Laura Berman, is to keep it in check. There's always a role for pornography and for fantasies, if it's used to the benefit of the couple, Berman said.