Objective: To determine the impact of My Student Body (MSB)-Nutrition, an Internet-based obesity prevention program for college students. Participants: Three hundred and twenty ethnically diverse undergraduate students were randomly assigned to 1 of 3 conditions: MSB-Nutrition program, an on-campus weight management course, and a comparison group. Methods: Students completed baseline and follow-up surveys regarding their nutrition and physical activity behaviors, self-efficacy, stress, attitudes, and body weight. Results: Compared with the on-campus course and a comparison group, the MSB-Nutrition program increased fruit and vegetable consumption, reduced stress, and increased fruit and vegetable self-efficacy but had no significant effect on students' exercise self-efficacy, exercise behavior, or weight loss. Conclusions: The MSB-Nutrition program was effective in changing students' nutrition behaviors but had no effect on physical activity behaviors or weight loss. Suggestions for improving Internet-based interventions aimed at decreasing obesity rates among college students are offered. (Contains 2 tables and 1 figure.)