Previous observational studies conducted predominantly in white men before 1988 estimated that 20% to 40% of myocardial infarctions (MIs) are unrecognized. Recent data on the proportion of MIs that are unrecognized, especially in women and African-Americans, are largely unavailable. Participants in the Atherosclerosis Risk in Communities (ARIC) study were men and women, aged 45 to 65 years, who were free of clinically recognized coronary heart disease and electrocardiographic evidence of MI at baseline (n = 12,843). Three follow-up clinic examinations were conducted approximately 3, 6, and 9 years after baseline, and included a 12-lead electrocardiogram at rest. Electrocardiographic evidence of infarction was defined as the appearance between the baseline and subsequent examinations of a major Q wave or a minor Q wave with ischemic ST-T changes. Clinically recognized (hospitalized) MI events were also identified and validated. Incident unrecognized MI was defined as electrocardiographic evidence of MI before, or in the absence of, a clinically recognized MI during the follow-up period. Of 508 MIs, 20% were unrecognized (95% confidence interval 16% to 23%), with African-Americans having a slightly higher percentage (23%) than whites (19%). The percentage of unrecognized MIs in men and women was similar. The percentage of unrecognized MIs in the ARIC sample between 1987 and 1998 was slightly lower than previous estimates from other populations.