Patients with late potentials in the signal averaged electrocardiogram are more at risk of lethal arrhythmias in the period after acute myocardial infarction. To test the effects of thrombolysis on the incidence and evolution of late potentials, 158 consecutive patients were prospectively studied during the first 10 days after acute myocardial infarction. The study population consisted of two groups: 93 control patients treated conservatively and 65 patients treated with intravenous thrombolysis. Recordings of signal averaged electrocardiogram were obtained within two days and 7-10 days after infarction. The incidence of late potentials in the first two days after infarction was not significantly different in the thrombolytic and control groups (14% v 11·8%). By 7-10 days the incidence of late potentials among patients who underwent thrombolysis remained unchanged (14%); however, it increased significantly in the control group (11·8% to 22·5%). Thus thrombolysis seems to reduce the evolution of late potentials within 10 days of infarction. Because the risk of fatal arrhythmias is higher in patients with late potentials this study may partly explain the reduced mortality after thrombolysis.