Thrombolysis with urokinase preactivated prourokinase was performed in 74 patients. At control angiography, 90 min into thrombolysis, in 39 (52.7%) patients the infarct-related coronary artery was found patent. However, 26 out of the 39 patients (66.7%) with a patent infarct artery demonstrated angiographic signs of residual coronary thrombi. With an estimated average of 75 +/- 25% in both, patients with and without residual thrombus, the severity of the residual stenosis after thrombolysis was found to be independent from the presence of angiographic signs of residual coronary artery thrombosis. In patients with residual coronary thrombi, however, a trend towards poorer coronary perfusion was observed (50% vs 77% of the patients with prompt antegrade [TIMI grade III] perfusion [p less than 0.2]. Control angiography performed 24 to 36h after thrombolysis revealed reocclusion of the infarct artery in 26.3% of the patients with residual coronary thrombi, while none of the infarct vessels without angiographic signs of residual thrombosis had reoccluded (p less than 0.05). At follow-up angiography 17 to 23 days after thrombolysis this difference in the incidence of reocclusions had further increased to 43.8% of patients with vs 12.5% of patients without residual coronary thrombus (p less than 0.01). Thus, after thrombolysis, residual thrombotic material remaining in the infarct related vessel seems to be a major risk factor, indicating early reocclusion of the infarct artery.