Data from a randomized clinical trial were used to examine the extent to which a health promotion intervention affected changes in psychosocial constructs and if so whether these in turn explained changes in physical activity (PA). PA and psychosocial data on 878 adolescents (ages 11-15) recruited through primary care providers (age M = 12.7 years, SD = 1.3; 58% white non-Hispanic) were measured at baseline, six and 12 months. Parallel process latent growth curve analyses found positive relationships between the growth trajectories of behavior change strategies, self-efficacy, family support, peer support and the growth trajectory of PA. However, mediation analyses did not reveal statistically significant intervention-mediated effects.