Information was collected by mail survey about myocardial infarction (MI), use of female hormones after menopause, and coronary risk factors 121,964 registered nurses ages 30-55 years. One hundred twenty-three women with a known type of prior menopause reported hospitalization for MI. Overall, use of female hormones by these women was very similar to that of control women matched for age and type of menopause. Compared with nonusers, the relative risk (RR) for women who had ever taken female hormones was 0.9 (95% confidence limits 0.6-1.2), and for current users the RR was 0.7 (0.5-1.1). For women with bilateral oophorectomy, the RR for current users was 0.4 (0.2-0.8). These data imply that, at present, a decision to prescribe postmenopausal hormones should be based primarily on weighing possible benefits from the relief of menopausal symptoms against unknown or suspected risks of other diseases, particularly uterine cancer in women with an intact uterus.