BACKGROUND AND OBJECTIVES: Early resolution of ST-segment elevation improves the short and long-term mortalities in acute myocardial infarction (AMI). However, the correlations between the ST segment resolution and microvascular integrity, or functional recovery of the left ventricle, were not explored. SUBJECTS AND METHODS: The study population consisted of 42 AMI patients who received thrombolytic therapy (35 male, 54+/-11 years) and consecutive successful percutaneous coronary intervention (PCI) (<30% of residual stenosis, recovered TIMI 3 flow) within 7 days. The coronary flow reserve (CFR) was measured at the segment just distal to the angioplasty site using intracoronary Doppler wire following a successful PCI. Electrocardiograms (ECG) were evaluated before, and within 90-150 minutes after, of the thrombolytic therapy. The percentage change in the ST segment resolution, from the baseline to follow-up, was categorized into complete resolution (70%, n=24), partial resolution (30% to <70%, n=10) and no resolution (0% to <30%, n=8). The ejection fraction (EF) and regional wall motion score indices (RWMSI) were assessed by 2D-echocardiography before, and following the PCI (9+/-5 months). RESULTS: Complete ST segment resolution was observed in 57%, partial resolution in 24%, and no resolution in 19% of patients. The CFR was significantly higher in the complete resolution group than in the no resolution group, and the minimal coronary vascular resistance index was significantly lower in the complete resolution group than in the no resolution group (2.1+/-0.5 vs. 1.4+/-0.4, p=0.006; 2.31+/-0.99mmHg sec cm-1 vs. 3.84+/-2.19mmHg sec cm-1, p=0.035, respectively). The changes in the EF and RWMSI were significantly better in complete resolution group than in the no resolution group (10+/-9% vs. 0+/-5%, p=0.028; -1.03+/-0.50 vs. 0.24+/-0.66, p=0.004, respectively), and the left ventricular end diastolic and systolic volume indices were significantly increased in the no resolution group at follow-up (42.0+/-14.8 mL/m2 vs. 55.8+/-18.1 mL/m2, p=0.006; 23.2+/-10.9 mL/m2 vs. 30.5+/-15.9 mL/m2, p=0.039, respectively). CONCLUSION: In the AMI patients, with a recovered TIMI 3 flow, following thrombolysis and successful elective PCI, there were differences in the coronary flow reserve according to the degree of early resolution of the ST segment. The patients with an early complete resolution of the ST segment showed the most favorable preservation of the microvascular integrities and improvement of the left ventricular function. The degree of early resolution of the ST segment might be a useful indicator for the prediction of left ventricular functional changes at follow-up.