We examined sources of variability in stature, body mass, and body mass index (BMI) in families of black and white elementary schoolchildren from Philadelphia, Pennsylvania. The sample consisted of 445 black and 379 white children, 7-13 years old, and their parents (total n = 2016). The sample was distributed among 596 nuclear families, each representing an independent pedigree. Maximum-likelihood-based variance decomposition methods were used to simultaneously estimate ethnic group-specific effects of genes, sex, age by sex, and unmeasured environmental factors on stature, body mass, and BMI. Likelihood ratio tests were performed to assess the significance of h2 estimates and differences in sigma g and sigma e between black and white families. Genes account for moderate proportions of the phenotypic variance (h2) of these traits in black and white children. In black and white children, respectively, h2 estimates were 0.37 and 0.53 for stature, 0.37 and 0.31 for body mass, and 0.38 and 0.24 for BMI (p < 0.0005). Although the differences in h2 between ethnic groups were not significant (stature, p = 0.23; body mass, p = 0.49; BMI, p = 0.14), black children exhibited a significantly greater total residual phenotypic standard deviation (sigma e and sigma g) in body mass and BMI and a significantly greater sigma e for stature compared with white children. The larger residual phenotypic variance in the black sample is likely due to exposure to unmeasured environmental factors that are not accounted for in this model. Given that sigma g for stature is not significantly different between ethnic groups, the slightly lower estimates in black children are due to the increased contribution of the environment to the phenotypic variance in this trait.