BACKGROUND: Several cohort studies in populations without coronary heart disease have demonstrated that up to 40% of incident myocardial infarctions are clinically unrecognized. OBJECTIVE: To determine the incidence of unrecognized myocardial infarction in women with coronary heart disease in the Heart and Estrogen/progestin Replacement Study (HERS). DESIGN: Randomized, double-blind, placebo-controlled trial of conjugated estrogens plus medroxyprogesterone or identical placebo with 4.1 years of follow-up. SETTINGS: Outpatient and community settings at 20 U.S. clinical centers. PATIENTS: 2763 postmenopausal women younger than 80 years of age with coronary heart disease and an intact uterus. MEASUREMENTS: Annual electrocardiograms were obtained for all participants during follow-up (mean, 4.1 years) and were evaluated by using the NOVACODE computer algorithm and visual confirmation. A total of 13 715 electrocardiograms were obtained. Suspected unrecognized myocardial infarctions were investigated by comparing a participant's previous surveillance electrocardiograms with the electrocardiograms obtained from all of her intervening hospitalizations. Characteristics of patients with recognized and unrecognized myocardial infarction were compared. RESULTS: Among the 256 nonfatal myocardial infarctions, 11 were unrecognized (4.3% [95% CI, 2.2% to 7.6%]). Seven occurred in women assigned to placebo and 4 occurred in women assigned to hormone therapy (P > 0.2). Women with unrecognized myocardial infarction were less likely to have diabetes mellitus or previous angina and were more likely to have had previous bypass surgery compared with women who had clinically recognized myocardial infarction. CONCLUSION: The incidence of unrecognized myocardial infarction in women with coronary disease was far lower than that observed in previous studies of populations without coronary heart disease.