Call it crazy, paranoid or cynical, but the next time you peruse the personals on Craigslist or scan profiles on MySpace, consider this: There's a good chance you just ran into a cheater.

Just as purchasing concert tickets or checking baseball scores has become as simple as logging onto a computer, infidelity is a simple keystroke away.

Cheating is on the rise because technology eases the search to find a willing partner, according to therapists, researchers and relationship experts. The unfaithful no longer have to scour bars or cultivate workplace relationships. Cheating has increased along with the growing use of text messaging and cell phones, chat rooms and online dating sites, some exclusively targeting the polygamous.

"The Internet has greatly removed the barriers," says Ruth Houston, founder of Infidelityadvice.com and author of "Is He Cheating on You? 829 Telltale Signs" (Lifestyle Publications, 192 pp., $29.95). "If you are a married person who wants to cheat, you can now go online and maintain an affair even while your spouse is in the room. Everything has changed."

Jill, 45, an elementary school teacher from Mountain View who asked that her last name not be used, learned of her partner's infidelity when she came across his open e-mail account, which he had failed to log off on their home computer. She was shocked to read that he had done "everything from soliciting hookers to making dates with others" via the Internet, she says. "I saw that he does this all day at work. I even posed as someone he had been conversing with, and he e-mailed me 30 times in one day!"

When Jill revealed her identity, he downplayed his online trawling, which "ruined our romance," she says.

No reliable figures exist on the increase in cheaters who use technology, but computer forensics expert John Lucich says the rise is undeniable. The president of Network Security Group, a firm in Union, N.J., hired for computer-related legal issues, says that 95 percent of the cases his company handles involve men and women who set up secret e-mail accounts for the purpose of cheating.

Online dating sites play a key role in connecting people searching for extracurricular activities. While mainstream services such as Match.com and Yahoo Personals ban married people from posting profiles, the dating sites can't stop users from lying. Other companies are happy to pick up the slack.

Private Affairs (www. philanderers.com), an online dating site based in Toronto, targets users looking for what it calls EMRs, or extramarital relationships. Another service, Ashley Madison Agency (www.ashleymadison.com), boasts 1.03 million members in the United States, Canada and the United Kingdom. With its tag line "when monogamy becomes monotony," the company, also founded in Toronto, has seen its membership double annually, says operations director and founder Darren Morgenstern.

"We're finding that it's just not going away," he says. "People are looking at the plausibility of using the Internet to have an affair, and it just works for them."

Once the connection is made, technology also helps the affair to thrive. Cell phones and PDAs give cheaters the chance to communicate privately and coordinate with their side dish.

Caryn, 37, a West Valley College student from Morgan Hill, knows this all too well. Like many wired people in Silicon Valley, she used to contact a former boyfriend almost exclusively on his cell phone.

"After several months, I found out he was married," says Caryn, who also asked that her last name not be used. "Much later, he even informed me that on several occasions I had even paged him during his marriage counseling sessions."

Statistics on cheating vary widely because of the way pollsters word questions, says Infidelityadvice.com's Houston. The data also is muddied by dishonest responses. And as people debate the definition of sex, they similarly debate the definition of cheating.

Sexologist Shere Hite in 1988 shocked Americans when she reported that up to 70 percent of women married five or more years have sex outside of marriage. Other surveys have concluded that anywhere from 38 million to 53 million men in the United States have cheated on their wives at least once, Houston says.

But such "studies," as well as research reported in popular magazines and advice columns, often inflate figures, according to Tom W. Smith of the National Opinion Research Center at the University of Chicago. His 2004 study, "American Sexual Behavior," which polled more than 10,000 people over 22 years, found that 22 percent of married men and 15 percent of married women have cheated at least once.

Technology has helped the cause, prompting the curious to make the jump from fantasy to philandering, says Brian Person, a marriage and family therapist in Los Altos.

"Some people, given the proper social boundaries, would be less likely to cheat than they are now," he says.

Network Security Group's Lucich is convinced that the rise in advertising and e-mail spam that hype cheating sites entice people to cross those boundaries, he says.

"I truly believe that there are people out there who have not thought about infidelity and then get spam messages or hear about online cheating and dating sites on the radio," says Lucich, whose book "Cyber Lies" (StarPath, 212 pp., $35) details how to easily check a partner's cell phone or computer to discover if he or she is cheating. "In a weak moment, they say, 'Let's just take a peek.' Then they start going further and further, and the next thing you know, they're cheating."

There is some small consolation in the rise of high-tech infidelity, Houston says, because cheaters are often unaware that they have left evidence of their affairs on their PCs or cell phones. E-mails are reportedly how Christie Brinkley found out her spouse was cheating on her with a local teenager.

"There are programs you can put onto a computer so you can see everything your mate is doing online," Houston says. "You can even put a GPS device in your mate's car to find out where they are going. It might be easier to cheat, but it's also a lot easier to get caught."