Adolescent girls with body dissatisfaction (N = 481, SD = 1.4) were randomized to a dissonance-based thin-ideal internalization reduction program, healthy weight control program, expressive writing control condition, or assessment-only control condition. Dissonance participants showed significantly greater decreases in thin-ideal internalization, body dissatisfaction, negative affect, eating disorder symptoms, and psychosocial impairment and lower risk for eating pathology onset through 2- to 3-year follow-up than did assessment-only controls. Dissonance participants showed greater decreases in thin-ideal internalization, body dissatisfaction, and psychosocial impairment than did expressive writing controls. Healthy weight participants showed greater decreases in thin-ideal internalization, body dissatisfaction, negative affect, eating disorder symptoms, and psychosocial impairment; less increases in weight; and lower risk for eating pathology and obesity onset through 2- to 3-year follow-up than did assessment-only controls. Healthy weight participants showed greater decreases in thin-ideal internalization and weight than did expressive writing controls. Dissonance participants showed a 60% reduction in risk for eating pathology onset, and healthy weight participants showed a 61% reduction in risk for eating pathology onset and a 55% reduction in risk for obesity onset relative to assessment-only controls through 3-year follow-up, implying that the effects are clinically important and enduring. (Contains 2 figures and 2 tables.)