To quantify the effects of early reperfusion on the size of infarcts, an enzyme indicator was developed: myocardial creatine kinase (CK) release rate (kr), based on a compartmental kinetics model. In 59 patients with acute myocardial infarction (MI) who received intracoronary thrombolysis therapy in the acute phase, the kr showed a good correlation with the flow condition of infarct-related coronary artery and the time required to reach peak enzyme activity. Apparent serum CK disappearance rate (kd') was estimated by using the method of Norris. The kd' was significantly underestimated in patients without reperfusion, suggesting the presence of prolonged enzyme release from the infarcted area. In 22 of 59 patients, who had a first acute anterior MI (left anterior descending arterial lesion), the correction of cumulative enzyme release by myocardial enzyme release (kr) resulted in a closer correlation with chronic phase left ventricular function. Thus, kinetic analyses of serum enzyme release provide a useful means to estimate the infarct size during intracoronary thrombolysis therapy.