Thrombolytic treatment for acute myocardial infarction increases the risk of subsequent reocclusion of the infarct related artery. The efficacy and safety of repeat thrombolytic treatment was assessed in 31 patients treated with streptokinase (n = 13) or tissue plasminogen activator (n = 18) a median of five days (1-716) after the first infusion. The indication for readministration was prolonged chest pain with new ST segment elevation. Efficacy was assessed by infarct artery patency at angiography at a median of eight days after readministration in 22 patients and by non-invasive criteria in 23 patients (reperfusion was deemed to be likely if serum creatine kinase was not increased or reached a peak less than 12 hours after infarction). Angiography showed patency of 70% of the infarct arteries after readministration of streptokinase and of 75% after tissue plasminogen activator. The corresponding patency rates assessed noninvasively were 73% and 75%. Reinfarction was prevented in nine (29%) patients. Allergic reactions occurred in four of eight patients who received streptokinase twice (plasmacytosis and acute reversible renal failure developed in one patient). Two patients had major bleeding and two minor bleeding, all after tissue plasminogen activator, and one of them died of cerebral haemorrhage. Repeat thrombolytic treatment results in late patency rates similar to the rates after the initial administration. Allergic reactions were common in those treated twice with streptokinase.