Reperfusion of ischemic myocardium has been reported to increase the cumulative creatine klnase activity in plasma per gram of Infarcted myocardium as assessed with the method of Shell et al. In an attempt to find out whether infarct size assessment using the method of Witteveen et al was affected by reperfusion, the relation between enzymatic infarct size was analyzed using Witteveen's method and left ventricular (LV) function parameters in 266 patients with first acute myocardial infarction randomized to Intracoronary thrombolysis (n = 134) or conventional therapy (n = 132). Compared with patients allocated to conventional therapy, patients allocated to intracoronary thrombolysis had smaller enzymatic infarct size by 29% (p < 0.001), smaller LV end-diastolic and end-systolic volume indexes by 10% (p < 0.05) and 20% (p < 0.005), respectively, and higher LV ejection fraction (55 ± 1 % vs 49 ± 1 %; p < 0.001). The beneficial effects of thrombolytic therapy on LV performance were closely associated with thromboiysis-induced limitation of infarct size. The dependence from infarct size of LV end-diastolic volume, LV end-systolic volume, and ejection fraction was not different in the 2 therapy groups. It was concluded that Witteveen's method of infarct size assessment is not influenced by the presence of reperfusion. Therefore, this method was recommended for trials on recanalization in patients with acute myocardial infarction.