As an early marker of outcome, the sum of ST-segment elevation resolution between the electrocardiogram before and 3 hours after initiation of thrombolysis was investigated in 77 patients with acute myocardial infarction. Prospectively, three groups were defined according to complete (> or = 70%, n = 34), partial (< 70% to > or = 30%, n = 26), or no (< 30%, n = 17) ST resolution. There were considerable differences in the enzyme-determined infarct size (alpha-hydroxybutyrate dehydrogenase release for complete, partial, and no ST resolution: 529 +/- 397 IU/L, 689 +/- 484 IU/L, and 1293 +/- 742 IU/L, respectively; p = 0.0001) and the angiographic left ventricular function 1 week later (ejection fraction 58% +/- 10%, 53% +/- 13%, and 43% +/- 12%, respectively, p < 0.01; regional dyssynergic area 24 +/- 19, 39 +/- 23, and 50 +/- 21 U2, respectively, p < 0.01). Early reperfusion as assessed by creatine kinase release measured in 15-minute intervals was 90%, 65%, and 18%, respectively (p = 0.0001). Differences in degrees of ST-elevation resolution at 3 hours may help facilitate timely screening of patients for appropriate therapeutic intervention. Patients with complete ST resolution may be considered for early discharge, and patients with no ST resolution may be candidates for an early invasive approach or additional thrombolytic therapy.