Abstract Objective: To investigate the association between annual change in physical activity and subsequent annual change in weight status in adolescents over a 4 year period.

Design: Longitudinalstudy design with annualdata collection over 4 years. Setting: Suburban school district in Pittsburgh, PA. Participants: 1245 adolescents aged 11-16 years. Main outcome measures: Physical activity and body mass index (BMI) percentile. Analysis: Generalized estimating equation (GEE).

Results: 994 participants (53% male) were included in the analyses. The longitudinal associations between changes in physical activity and BMI% were mostly small and non-significant. However, compared with those who did not change their physical activity, a large annual increase in physical activity (>10 hr/wk) was associated with a significant increase in BMI%. Results were broadly similar for total and vigorous physical activity, boys and girls and for healthy and unhealthy (>85th percentile) BMI%.

Conclusions and Implications: The role of physical activity in the development of obesity during adolescence therefore remains unclear. Further work is needed to elucidate the role of physical activity in the preventionof obesity.