Objective: Test the hypothesis that reductions in thin-ideal internalization and body dissatisfaction mediate the effects of a dissonance-based eating disorder prevention program on reductions in eating disorder symptoms over 1-year follow-up. Method: Data were drawn from a randomized effectiveness trial in which 306 female high school students (mean age = 15.7 years, SD = 1.1) with body image concerns were randomized to the 4-session dissonance-based prevention program or an educational brochure control condition, wherein school counselors and nurses were responsible for participant recruitment and intervention delivery. Results: Dissonance-intervention participants showed greater reductions in thin-ideal internalization, body dissatisfaction, and eating disorder symptoms; change in thin-ideal internalization predicted change in body dissatisfaction and symptoms; change in body dissatisfaction predicted change in symptoms; and all indirect effects were significant. Change in thin-ideal internalization fully mediated the effects of intervention condition on change in body dissatisfaction and partially mediated the effects on symptoms; change in body dissatisfaction partially mediated the effect of intervention condition on change in symptoms. Conclusions: Findings provided support for the intervention theory of this eating disorder prevention program over longer term follow-up, extending the evidence base for this effective intervention. (Contains 2 footnotes, 2 tables, and 4 figures.)