BACKGROUND Although Torsades de pointes (TDP) is a well known fatal complication of long QT syndrome (LQTS), predictors of TDP in bradyarrhythmia-induced LQTS have not been well defined. We sought to find the electrocardiographic predictors of TDP inpatients with acquired atrioventricular block and QT prolongation. METHODS We analyzed 12-lead electrocardiogram (ECG) of 11 patients who had atrioventricular block (AVB) and experienced episodes of TDP. The ECG repolarization parameters of TdP patients were compared with those of 44 age- and sex- matched controls. RESULTS From 2004 to 2010, a total of 11 patients experienced TDP during AVB. Most of the patients were female (81% [9/11]) and the mean age was 66.6 years old. TDP was initiated by PVCs with short-long-short sequence of activation in 7of 11 patients. The average cycle length of the long sequence was 2.2짹0.3 times longer than the cycle length of short sequence. Patients with TDP showed a significantly longer QTc interval (591.9짹42.5 vs. 461.1짹72.2, p= 0.001) compared with those without TdP. The best cut-off value of QTc interval for prediction of TdP was 540ms (91% sensitivity, 84% specificity, and AUC=0.932, 95% CI = 0.830 to 0.982, p<0.001). In morphology analysis, patients with TDP showed higher prevalence of notched T wave (100% [11/11] vs. 25% [11/44], p=0.001) and T wave alternans (36.4% [4/11] vs. 0% [0/44], p=0.001). Presence of notched T wave in any lead was most sensitive predictor with sensitivity of 100% and specificity of 75%. Presence of macroscopic T wave alternans was most specific predictor with sensitivity of 36.4% and specificity of 100% CONCLUSIONS Analysis of ECG repolarization parameters may be useful to define the high risk group of TDP in patients with acquired long QT syndrome.