Objective-To identify predictors of becoming eating disordered among adolescents. Design-Prospective cohort study. Setting-Self-report questionnaires.Subjects-Girls (n=6916) and boys (n=5618), aged 9 to 15 years at baseline, in the ongoing Growing Up Today Study (GUTS).Main Exposures-Parent, peer, and media influences.Main Outcome Measures-Onset of starting to binge eat or purge (ie, vomiting or using laxatives) at least weekly.Results-During 7 years of follow-up, 4.3% of female subjects and 2.3% of male subjects (hereafter referred to as "females" and "males") started to binge eat and 5.3% of females and 0.8% of males started to purge to control their weight. Few participants started to both binge eat and purge. Rates and risk factors varied by sex and age group (<14 vs ≥14 years). Females younger than 14 years whose mothers had a history of an eating disorder were nearly 3 times more likely than their peers to start purging at least weekly (odds ratio, 2.8; 95% confidence interval, 1.3-5.9); however, maternal history of an eating disorder was unrelated to risk of starting to binge eat or purge in older adolescent females. Frequent dieting and trying to look like persons in the media were independent predictors of binge eating in females of all ages. In males, negative comments about weight by fathers was predictive of starting to binge at least weekly. Additional Contributions: S. Bryn Austin, ScD, and Ruth Streigel-Moore, PhD, provided comments on preliminary analyses.Author Contributions: Dr Field had full access to all of the data in the study and takes responsibility for the integrity of the data and the accuracy of the data analysis. Study concept and design: Field, Camargo, and Laird. Acquisition of data: Field and Camargo. Analysis and interpretation of data: Field, Javaras, Aneja, Kitos, and Taylor. Drafting of the manuscript: Field, Kitos, and Taylor. Critical revision of the manuscript for important intellectual content: Field, Javaras, Aneja, Kitos, Camargo, Taylor, and Laird. Conclusions-Risk factors for the development of binge eating and purging differ by sex and by age group in females. Maternal history of an eating disorder is a risk factor only in younger adolescent females.Concerns about weight and body shape are common in preadolescents and adolescents 1,2 and are probably related to the development of unhealthy weight control behaviors and binge eating. However, because of the lack of large prospective studies of non-treatmentseeking samples, little is known with certainty about the development of disordered eating (eg, engaging in bulimic behaviors) or eating disorders.Although there have not been many large prospective studies of disordered eating (ie, engaging in binge eating or purging) or full-criteria eating disorders 3 among non-treatmentseeking samples, a variety of risk factors for disordered eating and eating disorders have been proposed. The support for some of these purported risk factors is stronger than for others. 4 The risk factors with the strongest support, which a...