Objectives. The aim of this study was to investigate the significance of further ST elevation that occurs during the 1st h of thrombolytic therapy before the expected resolution.Background. Early resolution of ST segment elevation is commonly accepted as a marker of clinical reperfusion during thrombolytic therapy far acute myocardial infarction. Using frequent electrocardiographic recordings, we observed in some patients further ST elevation that occurred during hour 1 of thrombolysis before the expected resolution.Methods. To investigate the significance of this pattern, we classified 177 consecutive patients with a first acute myocardial infarction into two groups: Group A, 98 patients with ST elevation ≥1 mm above the initial ST elevation during the 1st h of thrombolytic therapy, and Group B, 79 patients without this finding.Results. Although the presence or absence of additional ST elevation was not associated with a clinical or prognostic difference in patients with a first inferior or posterior acute myocardial infarction, its presence indicated a more favorable clinical outcome and prognosis in patients with anterior infarction. Among the patients with anterior infarction the 65 patients in Group A had a higher ejection fraction (44 ± 9% vs. 35 ± 11%, p < 0.01), less heart failure (15% vs. 35%, p = 0.02) and 8 lower in-hospital mortality rate (0% vs. 8%, p = 0.04) than did the 37 patients from Group B.Conclusions. Additional ST elevation early during thrombolytic therapy in patients with anterior infarction suggests a favorable clinical outcome and thus may be indicative of successful reperfusion.