Objective. To assess prospectively the influence of peers, parents, and the media on the development of weight concerns and frequent dieting. Design. Prospective cohort study. Setting. Questionnaires mailed annually to participants throughout the United States. Participants. One-year follow-up of 6770 girls and 5287 boys who completed questionnaires in 1996 and 1997 and were between 9 and 14 years of age in 1996. Main Outcome Measure. Onset of high levels of concern with weight and dieting frequently to control weight. Results. During 1 year of follow-up, 6% of girls and 2% of boys became highly concerned with weight and 2% of girls and 1% of boys became constant dieters. Peer influence was negligible. Independent of age and body mass index, both girls (odds ratio [OR]): 1.9; 95% confidence interval [CI]: 1.1-3.1) and boys (OR: 2.7; 95% CI: 1.1-6.4) who were making a lot of effort to look like same-sex figures in the media were more likely than their peers to become very concerned with their weight. Moreover, both girls (OR: 2.3; 95% CI: 1.1-5.0) and boys (OR: 2.6; 95% CI: 1.1-6.0) who reported that their thinness/lack of fat was important to their father were more likely than their peers to become constant dieters. Conclusions. Our results suggest that parents and the media influence the development of weight concerns and weight control practices among preadolescents and adolescents. However, there are gender differences in the relative importance of these influences.