In 76 supposedly healthy families, we investigated the familial resemblance of left and right carotid intima-media thickness (IMT) measured by B-mode ultrasonography and the impact of the common apolipoprotein E (apo E) polymorphism and the insertion/deletion polymorphism of the angiotensin-converting enzyme (ACE). Genetic factors accounted for about 30% of IMT variation. The insertion/deletion ACE polymorphism did not influence carotid IMT, whereas apoE polymorphism explained about 1.5% of only right carotid IMT variability independently of cholesterol levels. The apo epsilon2 and apo epsilon4 alleles were associated with lower right carotid IMT than was the apo epsilon3 allele. We conclude that genetic factors strongly contribute to IMT variability in healthy people and that the apo E polymorphism may be one of these factors.