More than one in four myocardial infarctions that occurred over 30 years in the Framingham Study were detected only because of routine biennial electrocardiographic examinations. Of these, almost half were completely silent. The fraction of infarctions unrecognized was higher in women (35 per cent) than in men (28 per cent). Such infarcts were uncommon in persons with angina and recurrent infarctions. Unrecognized infarctions were as likely as recognized ones to result in eventual death, heart failure, or strokes. Thus, unrecognized infarctions are common and have as serious a prognosis as typically symptomatic infarctions.