A multivariate normal model for pedigree analysis is applied to fasting total serum cholesterol and total serum triglyceride measurements on 771 individuals in 95 pedigrees from Rochester, MN. Univariate and bivariate analyses are carried out to determine to what extent the aggregation and coaggregation in families of these two traits may be attributed to shared genetic and environmental factors. Pedigrees were ascertained through a sample of schoolchildren enriched for those with serum cholesterol levels in the highest and lowest deciles of their age- and sex-specific distributions. Ascertainment is corrected for by conditioning the likelihood on the trait values of the probands. Univariate results confirm the findings of previous studies indicating that familial aggregation of serum cholesterol and triglyceride levels is due both to shared genes and to shared environmental factors. Results of the bivariate analyses suggest that the coaggregation of cholesterol and triglyceride levels in these families is strongly influenced by both shared genes (pleiotropy) and shared environmental factors. These findings are consistent with our understanding of lipid metabolism and of specific environmental factors known to influence both traits.