Background: The purpose of this study was to identify pathways used by psychosocial factors to influence physical activity and bone health in middle-school girls. Methods: Baseline data from the Incorporating More Physical Activity and Calcium in Teens (IMPACT) study collected in 2001 to 2003 were used. IMPACT was a 1 1/2 years nutrition and physical activity intervention study designed to improve bone density in 717 middle-school girls in Texas. Structural Equations Modeling was used to examine the interrelationships and identify the direct and indirect pathways used by various psychosocial and environmental factors to influence physical activity and bone health. Results: Results show that physical activity self-efficacy and social support (friend, family engagement, and encouragement in physical activity) had a significant direct and indirect influence on physical activity with participation in sports teams as the mediator. Participation in sports teams had a direct effect on both physical activity (b= 0.20, P < .05) and bone health and (b=0.13, P < .05). Conclusion: The current study identified several direct and indirect pathways that psychosocial factors use to influence physical activity and bone health among adolescent girls. These findings are critical for the development of effective interventions for promoting bone health in this population.