PURPOSE: Studies estimating the contribution of physical activity (PA) development of body mass index (BMI) in critical periods of childhood warranted. Therefore, we have prospectively investigated this boys and girls of the KOALA Birth Cohort study, the Netherlands, in the around adiposity rebound (i.e., 4-9 years old). METHODS: PA was assessed children (231 boys, 239 girls) using accelerometers at the ages of 5 and and height and weight were measured at 5, 7, and 9 years. BMI z-scores calculated to standardize for age and gender. Leaner and heavier classified according to the 25th and 75th percentile of our study examine longitudinal relationships between PA and BMI z-scores, estimating equation analyses were performed, stratified for gender and weight status (leaner, normal weight, and heavier children). RESULTS: In children, an increment of 6.5 minutes of moderate to vigorous physical (MVPA) was related to a subsequent decrease of 0.03 BMI z-scores both in (95% CI = - 0.07 to - 0.001) and girls (95% CI = - 0.05 to - 0.002). physical activity was also associated with a decrease of BMI in heavier not girls. In normal weight children, MVPA was associated with decrease boys but not girls. CONCLUSION: Increments of MVPA were associated with in BMI z-score in heavier children, both boys and girls. Promoting MVPA remain a major prevention vehicle for improving body composition in 4-9 children.