We studied 83 women younger than 46 years with acute myocardial infarction (MI) and 154 controls. There was a strong positive association between MI and the following: (1) age, (2) both oral contraceptive and noncontraceptive estrogen use, (3) cigarette smoking, and (4) the presence of predisposing medical conditions, eg, past MI, hypertension, and diabetes. ABO blood type and family history of arterial disease were also positively associated with MI. Whereas the risks for idiopathic stroke and venous thromboembolism have also been shown to be increased among oral contraceptive users, there is comparatively little correlation between these two illnesses and age or smoking in young women. The present study, taken together with previously published work, provides reasonable estimates of the vascular risks associated with oral contraceptive use.