Eating disorders, an important health problem among college-age women, may be preventable, given that modifiable risk factors for eating disorders have been identified and interventions have been evaluated to reduce these risk factors. To determine if an Internet-based psychosocial intervention can prevent the onset of eating disorders (EDs) in young women at risk for developing EDs. San Diego and the San Francisco Bay Area in California. College-age women with high weight and shape concerns were recruited via campus e-mails, posters, and mass media. Six hundred thirty-seven eligible participants were identified, of whom 157 were excluded, for a total sample of 480. Recruitment occurred between November 13, 2000, and October 10, 2003. Intervention A randomized controlled trial of an 8-week, Internet-based cognitive-behavioral intervention (Student Bodies) that included a moderated online discussion group. Participants were studied for up to 3 years. The main outcome measure was time to onset of a subclinical or clinical ED. Secondary measures included change in scores on the Weight Concerns Scale, Global Eating Disorder Examination Questionnaire, and Eating Disorder Inventory drive for thinness and bulimia subscales and depressed mood. Moderators of outcome were examined. There was a significant reduction in Weight Concerns Scale scores in the Student Bodies intervention group compared with the control group at postintervention (P < .001), 1 year (P < .001), and 2 years (P < .001). The slope for reducing Weight Concerns Scale score was significantly greater in the treatment compared with the control group (P = .02). Over the course of follow-up, 43 participants developed subclinical or clinical EDs. While there was no overall significant difference in onset of EDs between the intervention and control groups, the intervention significantly reduced the onset of EDs in 2 subgroups identified through moderator analyses: (1) participants with an elevated body mass index (BMI) (> or =25, calculated as weight in kilograms divided by height in meters squared) at baseline and (2) at 1 site, participants with baseline compensatory behaviors (eg, self-induced vomiting, laxative use, diuretic use, diet pill use, driven exercise). No intervention participant with an elevated baseline BMI developed an ED, while the rates of onset of ED in the comparable BMI control group (based on survival analysis) were 4.7% at 1 year and 11.9% at 2 years. In the subgroup with a BMI of 25 or higher, the cumulative survival incidence was significantly lower at 2 years for the intervention compared with the control group (95% confidence interval, 0% for intervention group; 2.7% to 21.1% for control group). For the San Francisco Bay Area site sample with baseline compensatory behaviors, 4% of participants in the intervention group developed EDs at 1 year and 14.4%, by 2 years. Rates for the comparable control group were 16% and 30.4%, respectively. Among college-age women with high weight and shape concerns, an 8-week, Internet-based cognitive-behavioral intervention can significantly reduce weight and shape concerns for up to 2 years and decrease risk for the onset of EDs, at least in some high-risk groups. To our knowledge, this is the first study to show that EDs can be prevented in high-risk groups.