OBJECTIVE: This study examined the prospective, potentially bidirectional association of aggressive behavior with BMI and body composition across childhood in three population-based cohorts. METHODS: Repeated measures of aggression and BMI were available from the Generation R Study between ages 6 and 10 years (N = 3,974), the Netherlands Twin Register (NTR) between ages 7 and 10 years (N = 10,328), and the Swedish Twin Study of Child and Adolescent Development (TCHAD) between ages 9 and 14 years (N = 1,462). In all samples, aggression was assessed with the Child Behavior Checklist. Fat mass and fat-free mass were available in the Generation R Study. Associations were examined with cross-lagged modeling. RESULTS: Aggressive behavior at baseline was associated with higher BMI at follow-up in the Generation R Study (β = 0.02, 95% CI: 0.00 to 0.04), in NTR (β = 0.04, 95% CI: 0.02 to 0.06), and in TCHAD (β = 0.03, 95% CI: -0.02 to 0.07). Aggressive behavior was prospectively associated with higher fat mass (β = 0.03, 95% CI: 0.01 to 0.05) but not fat-free mass. There was no evidence that BMI or body composition preceded aggressive behavior. CONCLUSIONS: More aggressive behavior was prospectively associated with higher BMI and fat mass. This suggests that aggression contributes to the obesity problem, and future research should study whether these behavioral pathways to childhood obesity are modifiable.