Familial aggregation of plasma lipid levels among 676 black juveniles, aged less than 20 years and residing in 202 households in East Baltimore, Maryland, was assessed by the intraclass correlation coefficient of residuals from sex-specific cubic regressions on age. Significant aggregation of both plasma cholesterol (r = 0.23, p less than 0.005) and plasma triglyceride (r = 0.27, p less than 0.005) levels were found. In addition, no effect on familial aggregation was found for each of several measures of family structure: number of juvenile household members, closeness in age, all children over 9 years of age, or shared surname. Two methods of classifying juvenile household size were compared. An association between classification method and juvenile household size was demonstrated (p less than 0.005) for the distributions of households and of children. The intraclass correlation coefficients for plasma cholesterol and plasma triglyceride for households classified by number of juveniles were higher than the coefficients for the house grouped by the number of juveniles admitted to the study, when compared for each classification number, 2 through 4. Evidence for an effect on familial aggregation of the socioeconomic status was shown by a comparison of intraclass correlation coefficients from households in East Baltimore with those from white and black households from the Columbia Medical Plan which have higher levels of occupation and education. Higher correlations were shown for the groups with higher socioeconomic status, regardless of race.