Effects of therapy with urokinase (UK) and with recombinant tissue plasminogen activator (rtPA) were compared in patients with acute myocardial infarction (AMI). To achieve homogenous therapeutic conditions the comparison was restricted to patients having their first AMI and to cases of clinically successful thrombolytic therapy (defined by non-invasive criteria, such as a 50% decrease in elevated ST-segment in the worst lead of a 12 lead ECG within 300 min after onset of thrombolytic therapy, complete pain resolution during thrombolytic therapy, and later confirmed by angiography 10 days after AMI). Effects of UK and rtPA on continuous multilead ST-segment analysis and cardiac proteins (creatine kinase and its isoenzyme CK-MB, aspartate transaminase and hydroxybutyrate dehydrogenase) were analyzed during 24 hours following onset of therapy. Continuous ST analysis showed a faster resolution of the elevated ST-segments after thrombolytic therapy with rtPA than with UK (p < 0.01). Accelerated idioventricular rhythms (P < 0.05) occurred sooner following rtPA than UK treatment. The wash-out of creatine kinase was increased (p < 0.01) after rtPA. Although both drugs induced comparable, angiographically controlled reperfusion, the results suggest that the process of reperfusion was accelerated during thrombolysis with rtPA compared to UK. Thrombolytic therapy of AMI with rtPA may hence improve myocardial salvage.