Identifying factors that contribute to students' behavior and weight improvements during school-based obesity prevention interventions is critical for the development of effective programs. The current study aims to determine whether the support and resources that adolescent girls received from their families were associated with improvements in physical activity (PA), television use, dietary intake, body mass index (BMI) and body composition during participation in New Moves, a school-based intervention to prevent obesity and other weight-related problems. Adolescent girls in the intervention condition of New Moves (n = 135), and one parent of each girl, were included in the current analysis. At baseline, parents completed surveys assessing the family environment. At baseline and follow-up, 9-12 months later, girls' behaviors were self-reported, height and weight were measured by study staff and body fat was assessed using dual-energy X-ray absorptiometry. Results showed few associations between family environment factors and girls' likelihood of improving behavior, BMI or body composition. These findings suggest that in general, school-based interventions offer similar opportunities for adolescent girls to improve their PA, dietary intake, and weight, regardless of family support.