Objective: Efficacy trials found that a dissonance-based eating disorder prevention program in which female high school and college students with body image concerns critique the thin ideal reduced eating disorder risk factors, eating disorder symptoms, and future eating disorder onset. The present effectiveness trial tested whether this program produces effects through long-term follow-up when high school clinicians recruit students and deliver the intervention under real-world conditions. Method: Female high school students with body image concerns (N = 306; M age = 15.7 years, SD = 1.1) were randomized to the dissonance intervention or an educational brochure control condition and completed assessments through 3-year follow-up. Results: Dissonance participants showed significantly greater decreases in body dissatisfaction at 2-year follow-up and eating disorder symptoms at 3-year follow-up than controls; effects on other risk factors, risk for eating disorder onset, and other outcomes (e.g., body mass) were marginal or nonsignificant. Conclusions: Although it was encouraging that some key effects persisted over long-term follow-up, effects were on average smaller in this effectiveness trial than previous efficacy trials, which could be due to (a) facilitator selection, training, and supervision; (b) the lower risk status of participants; or (c) the use of a control condition that produces some effects. (Contains 3 tables and 1 figure.)