Background: Although T wave alternans (TWA) has been known to be associated with the vulnerability of ventricular tachyarrhythmia (VT), none has presented that TWA immediately precedes or follows the spontaneous VT in an ambulatory setting. T wave peak to end interval (Tpte) is the parameter for the transmural dispersion of refractoriness. Methods and Results: We analyzed the electrograms (EGM) of the implantable cardioverter defibrillator (ICD) from 11 patients (Male:Female 9:2, Age 60.18짹12.33, Ischemic VT 38 %, Nonischemic VT 62 %). TWA, Tpte, QT interval, and RR intervals were measured from the EGM before and after appropriate ICD therapy (Clinical VT; CVT group, n=13), after ICD therapy to induced VT (Induced VT; IVT group, n=21), and before inappropriate ICD therapy (Control group, n=21). Results: 1) ��Tamplitude (TWA) was significantly higher in CVT group (1.16짹1.30 mV) than in IVT (0.30짹0.39 mV, p<0.01) or control groups (0.05짹0.10 mV, p<0.001). 2) Tpte was significantly longer in CVT group (16.02짹9.01 ms) than in IVT (10.02짹3.04 ms, p=0.018) or control groups (11.03짹2.01 ms, p=0.037). 3) Corrected QT was significantly shorter in CVT group (60.03짹30.02 msec) than in IVT (90.03짹10.01 ms, p<0.01) or control groups (100.04짹20.01 ms, p<0.01). RR interval was longer in CVT group (1090.01짹340.02 ms) than in IVT (800.30짹110.11 ms, p<0.05) or Control groups (920.12짹150.00 msec, p=0.09). Conclusion: TWA and Tpte of ICD-EGM are the significant predictors of development of VT, which indicates restitution property and transmural dispersion of refractoriness play important roles in the induction of VT.