Apolipoprotein E genotypes were measured in 83 patients with familial hypercholesterolaemia (FH) and in 175 blood donor controls. Following DNA extraction from peripheral blood, each sample was genotyped for the Apo E polymorphism by polymerase chain reaction. No significant differences were found in the levels of the ε2 and ε3 alleles between the two groups, while the ε4 allele was approximately twice as prevalent in the FH patients as in controls ( P = 0·006, df = 1). Of the FH patients, 8·4% were homozygous for the ε4 allele while this genotype was rare in controls ( P = 0·009, df = 1). These results suggest that the ε4 allele is over represented in the FH population and may contribute to increased cholesterol levels and consequent vascular disease.