Context: Previous research suggests substantial increases in genetic effects on disordered eating across adolescence. Unfortunately, these studies were cross-sectional and focused primarily on early (age 11 years) vs late (age 17 years) adolescence. Objective: To examine longitudinal changes in genetic and environmental influences on disordered eating across early, mid, and late adolescence. Design and Setting: Population-based study of female same-sex twins. Participants: Seven hundred seventy-two female adolescent twins from the Minnesota Twin Family Study assessed at ages 11, 14, and 18 years. Main Outcome Measures: Disordered eating symptoms (ie, body dissatisfaction, weight preoccupation, binge eating, and the use of compensatory behaviors) were assessed with the total score from the Minnesota Eating Behavior Survey. Results: Biometric model-fitting indicated significant changes in genetic and shared environmental effects across early to mid adolescence. Although genetic factors accounted for a negligible proportion (6%) of variance at age 11 years, genes increased in importance and accounted for roughly half of the variance (46%) in disordered eating at ages 14 and 18 years. Shared environmental influences decreased substantially across these same ages. Conclusions: Findings highlight the transition from early to mid adolescence as a critical time for the emergence of a genetic diathesis for disordered eating. The increase in genetic effects during this developmental stage corroborates previous research implicating puberty in the genetic etiology of eating disorders.