We studied familial aggregation patterns of plasma cholesterol and triglyceride concentrations in a sample of 242 families in a prepaid medical care setting. Three analytic methods of progressive sophistication--case finding, correlation, and multiple regression--were used. Case-finding analyses detected only slight association of elevated lipid levels in parent-offspring pairs. The results suggest that when probands are randomly selected from a free-living population, the value of screening families in which one member is shown to have elevated lipid levels is limited mainly to identifying "hypercholesterolemic" offspring of "hypercholesterolemic" fathers. Correlations of plasma lipid levels were significant for various pairs of first-degree relatives but negligible for spouses. Multiple regression also supported the predictive value of family members' lipid levels. Taken together, the results of these analyses provide strong evidence for familial aggregations of plasma cholesterol and lipid levels. The distinction of genetic and environmental contributions is a priority for future research.