Background We hypothesized that Purkinje potentials (PP) and their preferential conduction (PC) to the left ventricle (LV) identified during sinus rhythm (SR) are identical to those during ventricular tachycardia (VT) in patients with idiopathic left ventricular tachycardia (ILVT), and can be the appropriate target for radiofrequency catheter ablation (RFCA). Methods and Results In 14 patients with ILVT (Male 11, mean age 35짹13 years), we performed LV non-contact mapping (NCM) and compared PP and PC during sinus rhythm (SR) and VT. The target of RFCA was selected when the clear PP existed and the earliest activation site of PC during SR matched to that during VT. Results: All induced VTs (tachycardia cycle length 364.4짹64.7 ms, QRS width 109.8짹24.9 ms, 28.6% (4/14) induced by atrial pacing) showed clear PP and PC at LV inferior posterior septum. The PP and the earliest activation site of PC during VT were in close proximity (5.8짹8.2 mm) to those during SR. However, exit sites of VT were separated 30.2짹14.3 mm to apical region compared with PP. The interval between PP and QRS (PP-QRS) was longer during SR (32.6짹8.9 ms) compared with that of VT (22.3짹9.3 ms, p<0.02), and PP during SR had reverse polarity to those during VT (100%). RFCA targeting both PP and PC during SR eliminated VT in all cases (number of ablation 14.1짹7.1 per patient, procedure time 191.9짹62.5 min, fluoroscopic time 60.8짹31.2 min), and there was no recurrence within 15.3짹7.5 months of follow up. Conclusion NCM-guided PP and PC identification during SR matches well with those during VT and provides the effective targets for RFCA in patients with ILVT.