The history and clinical course of 175 patients aged 70 and older (Group A) with acute myocardial infarction (MI) was studied to determine the hospital mortality rate in this population group and to determine the clinical factors that influenced the hospital mortality rate. The mortality rate of a second group of 772 patients below age 70 years (Group B) admitted during the same time period was determined for comparison with the mortality rate of Group A. The hospital mortality rate in Group A (32%) was 3.9 times the hospital mortality rate in Group B (8.2%). Advanced age was associated with a significantly higher mortality rate, even when so called "good risk" patients from Group A were compared with patients in Group B. The major causes of death in this hospitalized group of patients aged 70 and older were shock (33.9%) and cardiac rupture (28.6%). Factors that significantly influenced hospital mortality rate in the elderly group were the development of shock, pulmonary edema on chest x-ray, a clinical diagnosis of congestive heart failure, and the development of complete heart block or new bundle branch block during the hospital course.