The complex interplay between genes and environment affecting body mass gain over lifecycle periods of risk is not well understood. We use longitudinal sibling cohort data to examine the role of shared household environment, additive genetic, and shared genetic effects on BMI and BMI change. In the National Longitudinal Study of Adolescent Health, siblings and twin pairs sharing households for > or =10 years as adolescents (N = 5,524; mean = 16.5 +/- 1.7 years) were followed into young adulthood (N = 4,368; mean = 22.4 +/- 1.8 years). Using a variance component approach, we quantified genetic and household effects on BMI in siblings and nonsiblings sharing household environments over time. Adjusting for race, age, sex, and age-by-sex interaction, we detected a heritability of 0.43 +/- 0.05 for BMI change. Significant household effects were noted during the young adulthood period only (0.11 +/- 0.06). We find evidence for shared genetic effects between BMI and BMI change during adolescence (genetic correlation (rho(G)) = 0.61 +/- 0.03) and young adulthood (rho(G) = 0.23 +/- 0.06). Our findings support a complex etiology of BMI and BMI change.