Prepubertal boys and pubertal girls and boys selected because of the occurrence of acute myocardial infarction in their father or mother were characterized by a low HDL cholesterol when compared to healthy controls. This observation extends our previous observations of low HDL cholesterol in adult relatives of CHD patients to the children of the same patients. The occurrence of an HDL abnormality in young children demonstrates that the low HDL cholesterol precedes the occurrence of cardiovascular disease. Low HDL cholesterol was associated with increased VLDL cholesterol in prepubertal children and pubertal boys without significant increase in VLDL triglycerides. Smoking and drinking habits, and physical activity which are environmental factors known to affect HDL did not differ between these children and the controls and cannot account for the observed differences. The results show that the low HDL cholesterol is detectable early in life in close relatives of CHD patients, persists throughout the lifespan and is possibly genetically determined.