One hundred and eighty-eight patients with evolving acute myocardial infarction were treated with intravenous streptokinase. Serial 12-lead electrocardiograms were recorded for 3 hours after treatment and inspected for rapid repolarization changes of the ST segment and T wave. Abrupt electrocardiographic repolarization changes were observed in 106 patients (56%) and were strongly predictive for an open infarct-related coronary artery at a mean of 6 days after treatment (predictive value = 0.92, sensitivity = 0.67). Abrupt electrocardiographic changes were not observed in 82 patients (44%). This absence was not a good predictor of an occluded infarct-related coronary artery (predictive value = 0.4). There was no relation between the presence or absence of abrupt electrocardiographic changes and global or regional left ventricular function after streptokinase treatment. Abrupt repolarization changes after thrombolytic treatment indicate a high probability of an open infarct-related artery. When abrupt repolarization changes do not occur, the patency of the infarct-related coronary artery cannot be predicted with accuracy. Serial electrocardiographic recordings do not provide sufficient information about coronary patency to eliminate the need for coronary arteriography.