The serum cholesterol and triglyceride levels and the incidence of ischemic heart disease were studied in 122 (55 men and 67 women) consecutive heterozygous familial hypercholesterolemic patients in the Hokuriku district of Japan. (1) The mean +/-SD of serum cholesterol level was 354.0 +/- 71.0 mg/100 ml, which was lower than those of the Western countries by about 60--70 mg/100 ml. (2) The mean +/-SD of serum triglyceride level was 116.5 +/- 54.0 mg/100 ml. (3) The average serum cholesterol values in the 20--50-year-old group showed no differences from those of the Western countries. However, in the above 50 years of age group the serum cholesterol levels were much lower than those in the United States. (4) The occurrence of ischemic heart disease in 83 heterozygous familial hypercholesterolemic patients was 43.3%. The incidence of myocardial infarction was 20.5%. Thus, familial hypercholesterolemia is as highly atherogenic as that of the Western countries even in Japan where the low incidence of coronary heart disease in the general population has been attributed to the low level of serum cholesterol.