In a series of 125 consecutive patients, enzyme determinations were performed at daily intervals for the first four days following acute myocardial infarction in order to assess the usefulness of enzymes in assessing prognosis. A direct relationship between amplitude of enzyme rise and early mortality was found; levels six times normal were associated with a hospital mortality of 56% (14 of 25). The incidence of arrhythmias was also correlated with enzyme activity, with 81.8% (102 of 125) of ventricular arrhythmias occurring in the group whose enzyme levels exceeded four times normal. In patients who developed congestive heart failure, the initial enzyme levels were four to five times normal. Cardiogenic shock was present in 10.4% (13 of 125) of patients and was not seen in any patient who did not have an enzyme rise at least five times normal.