Intracoronary thrombolysis is a logical therapeutic method and one of the challenging new treatments of acute myocardial infarction. However, a wide dose range of urokinase has been reported, and the optimal dose has not yet been established. In this study the fibrinolytic activity in patients with recanalized coronary arteries was compared with that in those with nonrecanalized arteries. The mean doses of urokinase in the recanalized and non-recanalized groups were 910,700 +/- 161,730 international units (IU) and 1,008,000 +/- 151,800 IU, respectively. The fibrinolytic activity was measured with alpha 2-plasmin inhibitor, alpha 2-macroglobulin, fibrinogen, plasminogen, and fibrin degradation products. No significant difference was observed in the fibrinolytic activity between the recanalized and nonrecanalized groups. Because the fibrinolytic activity in the two groups was thought to be activated sufficiently and to a similar degree, it appears that 1,000,000 IU of urokinase is adequate for intracoronary thrombolysis and larger doses cannot be expected to result in a higher rate of recanalization.