The ability of the electrocardiographic ST segment to predict successful reperfusion after thrombolytic therapy remains controversial. To evaluate whether angiographically determined reperfusion could be predicted from changes in ST-segment elevation, the sum of ST-segment elevation in affected leads of the electrocardiogram was compared before and after thrombolytic therapy in 53 patients with acute myocardial infarction (AMI). Reperfusion status of the infarct-related artery was determined angiographically < 8 hours from onset of symptoms. According to the Thrombolysis in Myocardial Infarction trial (TIMI) criteria, 33 patients had successful reperfusion (TIMI grade 2 to 3 flow) after thrombolytic therapy and 20 patients did not (TIMI grade 0 to 1 flow). Logistic multiple regression analysis showed that the proportional value for the shift in the sum of ST elevation, termed the "% ST change", was more strongly associated with reperfusion than the absolute measured difference in millimeters (chi-square = 11.34 vs 9.22). The entire spectra of sensitivities and specificities were determined to identify a level of the percent ST change with simultaneous high sensitivity and specificity. A 20% decrease in ST elevation provided such a level (88% sensitivity, 80% specificity). The positive and negative predictive values of a 20% decrease in ST elevation were 88 and 80%, respectively. These results suggest that a decrease of only 20% in the sum of ST elevation in the standard electrocardiogram after thrombolytic therapy is a useful noninvasive predictor of reperfusion status in patients with evolving AMI.