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 2-3 year follow-up than assessment-only controls. Dissonance participants showed greater decreases in thin-ideal internalization, body dissatisfaction, and psychosocial impairment than 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 2-3 year follow-up than assessment-only controls. Healthy weight participants showed greater decreases in thin-ideal internalization and weight than 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.