We reviewed the histories and laboratory data of 67 young patients with ischemic heart disease under the age of 40 years. Twenty-three cases were associated with hypercholesterolemia. Of these, 10 were of the familial type. Twenty patients had hypertension, while 14 were diabetics and 13 were heavy smokers. It is noteworthy that 13 of 17 patients with multi-vessel disease and hypercholesterolemia. Significant atherosclerotic coronary stenosis was demonstrated in 51 cases. The incidences of hypercholesterolemia, hypertension and diabetes mellitus in these 51 patients were not only significantly higher than those in age-matched normal health subjects and young patients with non-ischemic heart disease, but also higher than those in patients with ischemic heart disease over the age of 40 years. The results indicated that hypercholesterolemia is the most important risk factor for ischemic heart disease in young patients, although the three previously accepted risk factors (hypertension, diabetes mellitus and cigarette smoking) were all associated with the development of the disease. We could not find any obvious risk factor in 18 young patients with ischemic heart disease, including 12 cases of myocardial infarction. Myocardial infarction in the absence of any obvious risk factor occurred during strenuous physical exercise in 5 patients and following excessive alcohol consumption in 4.