Previous studies of idiopathic left ventricular tachycardia (ILVT, RBBB+left axis deviation morphology of QRS) demonstrated the importance of identifying the earliest Purkinje potential (PP) or late diastolic potential (DP) as the appropriate target for ablation. This study is to determine whether noncontact mapping system (NCM) is sensitive enough to detect PP or DP, which can be targeted for successful radiofrequency (RF) ablation. Methods: In four patients with ILVT (37짹15 y, M:F=3:1), a multielectrode array and a 7Fr mapping and ablation catheter were deployed in the LV via a retrograde transaortic approach. Isopotential color maps of ILVT were examined to determine the site of earliest activation (EA) during the end of diastole and the exit point (EX) subsequently depolarized the LV. PP was identified on the virtual electrograms (VEG) during sinus rhythm and VT. Pace mapping (PM) was performed. RF energy was focally applied to the EA or the area between EA and EX. Results: Cycle length of VT was 298짹39 ms and terminated with intravenous verapamil in all. During VT in 3 patients, EAs were identified at the sites where discrete PPs were shown on the VEG,. EA was followed by EX at the inferior septum in 2 and the apical septum in 1. The interval from EA to the onset of QRS was 50.8짹33.8ms. Distance between EA and EX was 21.3짹10.8 mm. No PP was seen in 1 patient who only showed EX at the mid-septum. PM performed at the EX showed perfect match (���11/12), but not at the EA (���10/12). RF (4.8짹3.1) applied at the EA in 3 and EX in 1 was successfully terminated VT. None had recurrence of VT after 10.3짹3.9 months of follow-up. Conclusions: In most patients with ILVT, NCM is useful to identify the site of Purkinje activation during VT, which can guide successful RF ablation