Although myocardial infarction has become increasingly well recognized, there continue to appear at necropsy myocardial infarcts, especially healed ones, which had been unrecognized clinically. It is also apparent that these unrecognized infarcts represent a considerable proportion of the myocardial infarcts at necropsy.1It was therefore felt worth while to study the case histories and the necropsy data on patients with myocardial infarction in order to determine the incidence of clinical nonrecognition and attempt to clarify some of the factors that obscured the diagnosis. Methods Cases for study were selected from the necropsies performed at the Mayo Clinic during the years 1953 and 1954. To be included in this study the case had to meet three prerequisites: (1) The medical record had to be apparently adequate with respect to the past history, physical examination, and details of the symptoms at the onset of the final illness; (2) the heart had