The frequency and severity of adverse effects resulting from the administration of streptokinase and alteplase were determined in 126 consecutive patients who received standard dosages of these agents for the treatment of acute myocardial infarction. Evaluation was based on patient assessment by nursing staff, physicians, and the investigators before, during, and after thrombolytic administration. Overall, adverse effects occurred in 15 (41.7%) of 36 patients receiving streptokinase and 12 (13.3%) of 90 receiving alteplase (p = 0.001). No major bleeding or neurologic events were documented. Minor bleeding occurred in 13.9% and 7.8% of streptokinase and alteplase recipients, respectively (p = 0.47), and hypotension in 8 (22.2%) and 5 (5.6%), respectively (p = 0.01). The frequency of hypotension associated with streptokinase was significantly higher than that with alteplase. Thrombolytic-induced hypotension was easily managed and was not associated with sequelae.