Serial estimations of total serum creatine kinase (S-CK) were made in 194 consecutive patients with acute myocardial infarction (AMI). By itself, the maximum CK value could not separate patients in terms of high and low mortality but when the maximum CK value was related to age for patients with and without a history of previous AMI, two subgroups became apparent, one with 46% mortality (high-risk group) and another with 6% (low-risk group) during the hospital stay plus the next 90 days. In 114 of the patients, infarct size could be calculated. A good correlation was found between maximum CK and calculated infarct size (r = 0.93). Calculated infarct size alone could not distinguish between high and low mortality but when it was related to age for patients with and without a history of previous AMI, two subgroups emerged, one with 43% mortality and another with 3% during the hospital stay plus the next 90 days. The incidence of ventricular tachycardia during the stay in the Coronary Care Unit did not differ between the two risk groups separated either by maximum CK value or calculated infarct size. However, the incidence of shock and severe left heart failure during the acute phase was higher in the high-risk groups.