The expression of ischemic heart disease was studied in a large kindred with familial hypercholesterolemia. Tendon xanthomas, multiple generation transmission, and the appearance of bimodality in the distributions of total and low-density lipoprotein cholesterol were found. The segregation ratio was 0.9 in females and 0.43 in males, a difference first apparent during adolescence. The upper quartile of total and low-density lipoprotein cholesterol contained all but two cases of ischemic disease, whereas the lower quartile of high-density lipoprotein cholesterol contained one half of the cases. The ratio of high- to low-density lipoprotein cholesterol (range, 0.06 to 1.6) was 0.20 in each patient with ischemic disease. The association of a low level of high-density lipoprotein cholesterol with ischemic disease persisted after adjustment for differences in other lipids and lipoproteins. A low level of high-density lipoprotein cholesterol, as well as a high level of low-density lipoprotein cholesterol, may influence the development of ischemic heart disease in this disorder.