Background Although T wave alternans (TWA) and T wave peak-to-end (Tpte) interval are associated with a vulnerability to ventricular tachyarrhythmias (VT), no previous reports have demonstrated that TWA immediately precedes spontaneous VT in the human ambulatory setting. Methods and Results We analyzed the stored electrograms from the implantable cardioverter defibrillators (ICD) of 74 patients (59 Male, 55.3짹12.2 years old). TWA (��T amplitude), Tpte interval, QT interval, and RR intervals on ICD electrograms were measured from magnified digital images; the electrograms were obtained immediately before spontaneous VT (VT_Clinical; n=73), or immediately after ICD shocks during artificially-induced VT (VT_Induced; n=74) or inappropriate shocks (Shock_Inapp; n=6). Results: 1. TWA were significantly greater in VT_Clinical (61.1짹26.3 關V) than in VT_Induced (36.2짹27.5 關V, p<0.01) or Shock_Inapp (34.7짹19.3 關V, p<0.001), but Tpte was not significantly different among VT_Clinic, VT_Induced, and Shock_Inapp. 2. In VT_Clinical, TWA were significantly greater in patients with ischemic VT (70.5짹27.4 關V) as compared to those with non-ischemic cardiomyopathy (51.3짹19.6 關V, p<0.05) or idiopathic VF (46.6짹27.4 關V, p<0.05). 3. In the same patients, the TWA for VT_Clinical (57.7짹27.0 關V) were significantly greater than those for VT_Induced (40.5짹23.8 關V, p<0.01). Conclusion TWAs measured from ICD electrograms immediately before the onset of spontaneous VT are significantly greater than immediately after inappropriate shocks or shocks during induced VT, especially in patients with ischemic cardiomayopathy. These findings indicate that repolarization alternans plays an important role in the induction of VT in humans.