BACKGROUND Long-term effects on the left ventricular (LV) function of acute ST re-elevation (STRE) during thrombolysis in ST segment elevation myocardial infarction (STEMI) patients have not been investigated. METHODS Patients with anterior STEMI treated with TNK within 12 h from symptom onset (n=191) were investigated. We compared the clinical, the electrocardiographic, and the angiographic data of patients with STRE (additional ≥2.0 mm ST elevation during the first 60 min, followed by >50% ST resolution, n=31) with those with conventional ST resolution (>50%, n=88) and those without ST resolution (<50%, n=72). We also compared the ejection fraction (EF) and LV volumes by echocardiography in the acute phase and at 12 months between the three groups. RESULTS Maximum ST elevation before TNK was higher in conventional ST resolution patients than in the other two groups (P=0.01) and additional STRE was 4.7±2.4 mm. Time from pain onset to TNK in STRE and ST resolution groups was similar but shorter than that in patients without ST resolution (P=0.01), whereas the levels of creatine kinase MB mass, EF, and LV volumes were comparable in the three groups. At 12 months, EF was also similar in patients with STRE than in those with ST resolution (51±8 vs. 53±12%, P=0.52), but LV systolic (60±24 vs. 53±12 ml, P=0.01) and diastolic volumes (124±39 vs. 101±31 ml, P=0.02) were significantly larger in the former. CONCLUSIONS STRE during lytic therapy is associated with greater ventricular dilatation at 12 months than conventional ST resolution, suggesting an impaired reperfusion process. These differences, however, are often not visible in the acute phase.