Purpose: The study evaluated whether targeted changes in factors influencing enjoyment of physical education (PE), physical activity enjoyment, and self-efficacy beliefs about participating in physical activity mediated the effect of the Lifestyle Education for Activity Program (LEAP) intervention on participation in physical activity. Methods: High schools (N = 24) paired on enrollment size, racial composition, urban or rural location, and class structure were randomized into control (N = 12) or experimental (N = 12) groups. Of the 4044 girls enrolled and eligible, 2087 (51.6%) participated in the measurement component of the study. There were 1038 girls in the control group and 1049 girls in the experimental group. Intervention: LEAP was a comprehensive school-based intervention emphasizing changes in instruction and school environment designed to increase physical activity among black and white adolescent girls. It was organized according to the Coordinated School Health Program and included a PE component with core objectives of promoting enjoyment of PE, physical activity enjoyment, and self-efficacy. Results: Latent variable structural equation modeling indicated that: 1) the intervention had direct, positive effects on physical activity and factors influencing enjoyment of PE, which subsequently explained the effects of increased physical activity enjoyment and self-efficacy on increased physical activity; and 2) an additional, indirect effect of physical activity enjoyment on physical activity operated by an influence on self-efficacy. Conclusions: Increases in enjoyment partially mediated the positive effect of the LEAP intervention. To our knowledge, we have provided the first experimental evidence from a randomized controlled trial linking increased enjoyment with increased physical activity among black and white adolescent girls. Copyright © 2005 by the American College of Sports Medicine.