Some of the Web sites that cater to girls and young women with the eating disorders anorexia and bulimia may worsen their prospects for recovery, a new survey suggests.

Reporting on a survey of eating-disorder patients, a group of Stanford University researchers said those who admitted using these sites were sick longer and spent less time on schoolwork than their counterparts who did not visit them.

Patients who visited pro-recovery sites as well as the pro-eating-disorder sites became hospitalized more often than those who took advantage of neither type of resource.

The research findings are published in Pediatrics, the journal of the American Academy of Pediatrics.

Anorexia nervosa is a psychiatric diagnosis that describes an eating disorder characterized by low body weight and body image distortion. Individuals with anorexia often control body weight by voluntary starvation, purging, vomiting, excessive exercise, or other weight control measures such as diet pills or diuretic drugs.

Bulimia is a psychological condition in which the individual engages in recurrent binge eating followed by intentional purging to compensate for the excessive intake of food and prevent weight gain.

Purging typically takes the form of vomiting; inappropriate use of laxatives, enemas, diuretics or other medication; and excessive physical exercise or fasting.

Many of the pro-ana and pro-mia sites offer acceptance to victims of eating disorders, typically teen-age girls, with "thinspirational" images of actresses, models and celebrities -- for example, the late Audrey Hepburn, who is said to have carried just 103 lbs. on her 5'7" frame and the late Princess Diana, whose struggle with bulimia was well-known.

While the sites may be well-intentioned, often encouraging treatment and support for recovery, they often include information on dangerous weight-loss or purging techniques as well.

There are currently more than 200 pro-ana and pro-mia Web sites, according to the report, which estimates that .5 to 1 percent of adolescent and adult women are anorexic, while 1 to 2 percent are bulimic.