They're called pro-eating-disorder Web sites. And many teens looking to lose weight — even those who don't need to — are logging on to these communities of individuals who engage in dangerous eating habits.

Yet many parents aren't aware that the sites exist or that their children are visiting them, researchers have found.

"Most parents would not endorse their child leaving the house at night, walking to an area of town that they themselves had never been in and meeting people that they themselves had never met," said the study author, Dr. Rebecka Peebles, an instructor in adolescent medicine at Lucile Packard Children's Hospital at Stanford University School of Medicine. "And I think that's actually what a lot of kids are doing online at home from their bedrooms."

Potentially dangerous venues include Web sites where people who engage in disordered eating gather to discuss their activities. A majority of these sites have sections where people share tips and techniques "that I would consider as a physician fairly harmful," Peebles said.

Complicating matters, sites that promote recovery from eating disorders can also be a source of weight-loss or purging techniques, the researchers found.

Peebles and her colleagues sent questionnaires regarding Internet use and eating-disorder information to the parents of almost 700 people who had been evaluated for an eating disorder at Lucile Packard Children's Hospital between 1997 and 2004. A total of 182 responded, including 76 teens and 106 parents.

Forty-one percent of the patients had visited a pro-eating-disorder Web site, while 36 percent had visited a pro-recovery Web site.

One of the problems, Peebles explained, is that even forums designed to disseminate helpful information are constantly changing and may contain content that wasn't the author's original intent.

"In our study, we did find that 96 percent of patients who visited (pro-eating-disorder) sites reported learning dieting tips and purging techniques from Web sites," she said. "However, nearly half of the people who visited pro-recovery Web sites also reported learning similar tips and techniques."

Fifty-two percent didn't know whether their children visited pro-eating-disorder Web sites, and 62.5 percent had no knowledge of pro-recovery sites. Half of parents of users of pro-eating-disorder Web sites either didn't know whether their child visited these sites while in treatment or thought their child did not visit these sites.

Peebles said the findings, published in the journal Pediatrics, underscore the need for parents to educate themselves about the Internet so they can make guide their teens' Web activity.

"I think a lot of parents are deciding by default not to be involved in their kids' online activities really, just because Google, in and of itself, is scary to them," she said.

"Most of our kids are more savvy surfers than we are," agreed Cynthia M. Bulik, professor of eating disorders at the University of North Carolina at Chapel Hill School of Medicine and director of the UNC Eating Disorders Program at UNC Hospitals. To help parents keep control, she tells them to "hone your own surfing skills" and "keep your family-access computer in a place where everyone can see what is being surfed."

Parents also need to talk to their children about what they find on the Internet, Bulik advised. And if they learn that their child is visiting eating-disorder Web sites, they should talk about it.

Bulik said, "I always tell parents, if your child comes home and says, 'I'm going on a diet' or 'I feel fat,' you should take it as seriously as if they were to come home and say, 'I'm trying my first cigarette' or 'I'm going to have a beer' — time to start the conversation!"