Although the genetics of relative weight have been investigated in several studies, most of these have been done primarily, if not exclusively, with whites. This study examined the heritability of body mass index (BMI) in 238 pairs of adolescent black and white male and female twins. BMIs were residualized for age and transformed to approximate normality. Hierarchically nested structural equation models were tested. An AE model (A = additive gene effects, E = unique environmental influences) in which the degree to which genetic and environmental factors influence BMI varies by race provided the best fit. Both the genotype and the environment exerted a greater influence on the BMI of black than white adolescents. Thus, although the variances in BMI are greater for blacks, the heritabilities were the same for blacks and whites. Implications for future research are discussed.