A family affected with hyperbetalipoproteinemia was studied. Two siblings, a boy 11 and a girl 9 years of age, with characteristic findings of homozygous type II hyperlipoproteinemia are described. The highly atherogenetic nature of this disorder is illustrated by the occurrence of progressive coronary atheromatosis in the boy, in whom two coronary arteriographic studies documented the development of progressive obstruction of two coronary arteries in the course of 2 years. During this interval angina pectoris developed followed by myocardial infarction, cardiomegaly and congestive heart failure. The sister has remained asymptomatic, with apparently normal coronary arteries at age 9 years. Phonocardiograms were suggestive of aortic valve involvement although no aortic valve gradient was demonstrated by cardiac catheterization.