Apolipoprotein E (apoE) polymorphism is a genetic determinant of plasma lipid levels and of coronary heart disease (CHD) risk. We determined the apoE phenotypes and plasma lipid levels in 1577 youths aged 3 to 18 years in 1980. The subjects were randomly selected from five areas of Finland. ApoE phenotyping was performed directly from plasma by isoelectric focusing and immunoblotting. The apoE allele frequencies in the population sample were epsilon 2 = 0.039, epsilon 3 = 0.767, and epsilon 4 = 0.194. There were no differences in the apoE phenotype distribution between East and West Finland or between sexes. The concentrations of serum total cholesterol, low density lipoprotein cholesterol, and apolipoprotein B increased with apoE phenotype in the order of E2/2, E3/2, E4/2, E3/3, E4/3, and E4/4. This increase was already seen in 3-year-old children; it was observed in both sexes, but was clearer in males than in females. The mean levels of high density lipoprotein (HDL) cholesterol, apolipoprotein A-I, triglyceride, Lp[a] lipoprotein, and the activity of lecithin:cholesterol acyltransferase did not differ between the apoE phenotypes. The observed differences in serum cholesterol remained fairly stable during the 6-year follow-up from 1980 to 1986, while the mean serum cholesterol concentration in the whole study population decreased by 6.3%. This study confirms the reported higher frequency of the epsilon 4 allele in Finns as compared to most other populations; this may contribute to the high rates of CHD in Finland as compared to most other populations. The results do not, however, explain the higher rate of CHD in East Finland in comparison to the western part of the country.