In 698 patients with suspected and definite acute myocardial infarction we tried to predict the severity of the infarction from clinical history and simple bedside evaluation soon after arrival in hospital. The severity of the infarction was judged from serum enzyme activity, 2-year survival, incidence and severity of congestive heart failure and incidence of severe ventricular arrhythmias during initial hospitalization. Entry characteristics which were positively associated with the severity of the infarction were intensity of pain, sign of congestive heart failure, high heart rate, ECG signs of acute myocardial infarction and presence of Q waves. Elderly patients and those with a history of hypertension also had a more severe clinical course.