Serial determinations of CPK enzyme were performed every 4 hours during a 72 hour period in 40 patients with acute myocardial infarction (AMI) admitted to the Coronary Care Unit in the first 6 hours (average 2.6) from the appearance of symptoms. The peak ratio of activity of CPK was 708 mU/ml +/- 48 E.S. as medium value in the whole group was reached in a medium period of 21,1 +/- 0,74 E.S. hours from the attack. Half value of the peak ratio activity was reached after a medium time of 19,1 +/- 1,0 E.S. hours. A significant statistical correlation between the CPK peak ratio and the prognostic index of Selvini et al. was found. The peak ratio resulted in 571 +/- 41 E.S. in patients with uncomplicated AMI, whereas in those with complications such as arrhythmias and heart failure the average value was 901 +/- 136 E.S. No significant correlation between CPK values and ST wave evolution of the ECG peak ratio of 1638 mU/ml was found; however, one patient who died of cardiac rupture showed a low level of 395 mU/ml. The diagnostic and prognostic value of the serial determination of CPK during the first 48 hours of a coronary attack is emphasized.