Of 708 myocardial infarctions among 5127 participants in the Framingham Study, more than 25 per cent were discovered only through the appearance of new diagnostic evidence during routine biennial electrocardiographic examinations. Of these unrecognized infarctions almost half were "silent," and the others caused atypical symptoms. The proportion of all infarcts that were unrecognized was higher in women and in older men. Such infarcts were uncommon in persons with angina. Unrecognized infarctions were as likely as recognized ones to cause death, heart failure, or strokes. Recurrent infarctions were more common in women with recognized than with unrecognized infarcts, but this difference was not present in men. Recurrent infarctions were more likely to be recognized than were first infarctions. We conclude that unrecognized infarctions are common and have as serious a prognosis as recognized infarctions.