One-hundred consecutive patients who were 35 years of age of younger underwent coronary arteriography after clinical myocardial infarction. Ninety-two percent were men. Four distinct subgroups were identified: Ninety-four patients (78 percent) had significant coronary artery disease (greater than 50 percent diameter narrowing of at least one major coronary artery), 20 (17 percent) had normal coronary arteries, 5 (4 percent) had major coronary arterial anomalies and 1 patient had coronary arteritis. Of the patients with obstructive coronary disease, risk factors were smoking in 89 percent, positive family history of coronary artery disease in 48 percent, hypertension in 21 percent and a history of lipid abnormality in 20 percent. Risk factors were distinctly less frequent in the groups without coronary atherosclerosis. In the group with coronary artery disease, the prevalence rate of one, two and three vessel disease was 32, 26 and 42 percent, respectively. Coronary arterial anomalies included anomalous origin of the left coronary artery from the pulmonary artery (three patients) and single right and single left coronary artery (one patient each). It is concluded that myocardial infarction before age 36 is a disease of men who smoke and who often have a family history of premature coronary artery disease. Twenty-two percent of patients will have normal coronary arteries, coronary arterial anomalies or coronary vasculitis. Coronary arteriography should be considered for patients who sustain a myocardial infarction before age 36 for purposes of diagnosis, management and prognosis.