Thrombin contributes to the pathogenesis of acute myocardial infarction and reocclusion after thrombolysis. Thrombolytic therapy is known to induce a paradoxic increase in thrombin generation. Specific thrombin inhibition enhances thrombolytic therapy in experimental models. The aim of this study was to determine thrombin generation in patients with acute myocardial infarction treated with rt-PA and conjunctive therapy with the specific thrombin inhibitor, recombinant hirudin. Thrombin generation was determined in 17 patients with acute myocardial infarction treated with front-loaded rt-PA (100 mg/90 min) and conjunctive therapy with recombinant hirudin (HBW 023 bolus 0.4 mg/kg, infusion of 0.15 mg/kg/h) over 48 hours. Mean free hirudin plasma levels of 1320–1545 ng/mL produced a stable anticoagulation with mean aPTT values between 63 and 81 seconds throughout the treatment period. Thrombin generation increased during thrombolysis, indicated by a transient elevation of prothrombin fragment 1.2 levels, which were 3.0 nmol/L at baseline, 11.1 nmol/L after 30 minutes, 8.3 nmol/L after 60 minutes, 3.1 nmol/L after 12 hours, and 1.5 nmol/L after 24 hours, respectively. In contrast, thrombin-antithrombin III complex levels during and after thrombolysis did not exceed the baseline level of 21.8 ug/L. Thrombin-hirudin complex levels increased constantly during the 48-hour treatment period from 3.1 ug/L at baseline to 64.2 ug/L. All patients had an open infarct vessel (TIMI 2/3 potency) after 36–48 hours. Thrombolysis with rt-PA is associated with a significant increase in thrombin generation, which is not blocked by r-hirudin, whereas circulating thrombin seems to be effectively inhibited by r-hirudin.