Objective: To identify factors associated with changes in physical activity in adolescent girls at risk for sedentary lifestyles and obesity. Design: A cohort study was performed with 201 high school girls recruited to participate in an evaluation study of a school-based obesity prevention physical education program. Three assessments were performed during an 8-month period. Main Outcome Measures: Associations between physical activity and a range of personal factors (self-acceptance, self-worth, athletic competence, body image, depressive mood, perceived benefits, enjoyment of physical activity, self-efficacy, and body mass index), behavioral factors (watching television and time constraints), and socioenvironmental factors (social support and costs/resources) were assessed. Results: The 2 strongest and most consistent factors associated with change in physical activity were time constraints and support for physical activity from peers, parents, and teachers. Measures assessing self-perceptions, global (ie, self-worth) and specific to physical activity (ie, self-efficacy to be physically active), were also associated with change in physical activity. For example, a decrease of 2.0 U for an adolescent's perceived time constraints (possible range, 3.0-12.0 U) would be expected to lead to an increase of 53 minutes of moderate to vigorous physical activity per week (95% confidence interval, 33-72 minutes). An increase of 2.0 U in perceived support for physical activity (possible range, 3.0-12.0 U) would be expected to lead to an increase of 35 minutes of moderate to vigorous physical activity per week (95% confidence interval, 13-56 minutes). An increase of 3.0 U on the self-worth scale (possible range, 5.0-20.0 U) might be expected to lead to an increase of 19 minutes of moderate to vigorous physical activity per week (95% confidence interval, 0-40 minutes). Conclusion: The effectiveness of interventions aimed at increasing physical activity among adolescent girls might be enhanced by engaging support from friends, family, and caring adults; addressing real and perceived time constraints; and helping adolescent girls feel more confident about themselves and their ability to engage in physical activity.