Background and Objectives Lethal ventricular arrhythmia (LVA), including ventricular tachycardia (VT) and ventricular fibrillation (VF) is usually developed within 48 hours after acute myocardial infarction (AMI). The mechanism responsible for the development of LVA after AMI is still unclear. Methods We measured the corrected QT interval (QTc) with Bazett's formula and interval from the peak to the end of the T wave (TpTe) which indicates myocardial transmural dispersion of repolarization in 46 patients with AMI. TpTe was also expressed as the relative to the duration of QTe (TpTe/QTe x 100%), and the corrected value for heart rate (TpTe/���RR). These parameters were obtained from all the 12-leads of ECG after arrival at the hospital, just before and after primary percutaneous coronary intervention (PCI), 4 hours, 24 hours, 48 hours, and 5 days later after primary PCI. Results The QTc significantly shortened from 437 짹 45 ms on admission to 428 짹 45 ms just after primary PCI (p=0.030). The TpTe/QTe x 100% shortened from 22 짹 5 % on admission to 20 짹 3 % after PCI (p=0.003). The TpTe/���RR shortened from 94 짹 26 ms on admission to 84 짹 18 ms after PCI (p=0.009). These parameters were not different between the data on admission and just before PCI. The Patients were divided into two groups based on the occurrence of VT during 48 hours after AMI as VT group (n=13) and non-VT group (n=33). The TpTe (V2, 83 짹 25 ms vs 108 짹37 ms, p=0.032, V3, 88 짹 25 ms vs 110 짹 32 ms, p=0.018), TpTe/QTe x 100% (V2, 21 짹 6 % vs 26 짹 8 %, p=0.017, V3, 22 짹 5 % vs 27 짹 7 %, p=0.014) and TpTe/���RR (V2, 87 짹 26 ms vs 117 짹 44 ms, p=0.04, V3, 94 짹28 ms vs 117 짹 37 ms, p=0.021) in V2 and V3 leads were significantly prolonged in the VT group compared with non-VT group just after PCI. Conclusion The mechanisms responsible for the development of LVA after AMI may be associated with both differences in local ventricular myocardial recovery times and transmural dispersion of repolarization. And Primary PCI may have important role in reducing the LVA after AMI.