INTRODUCTION Anatomic extent of the reentry circuit in idiopathic left ventricular tachycardia (ILVT, fascicular tachycardia with RBBB and superior axis deviation) has not been defined. HYPOTHESIS We hypothesized that entrainment technique could be used to delineate the anatomic reentry circuit in patients with ILVT. METHODS Ten consecutive patients(pts) with ILVT were included. Entrainment pacing was performed at the right ventricular apex, right ventricular outflow tract, mid left ventricular septum and peri-Hisian area(PHA) where His or bundle branch potentials were recorded. In 3 patients, electroanatomical mapping was performed to tag the location of the concealed entrainment and the earliest ventriuclar activation. RESULTS 1. During tachycardia, earliest ventricular activation was recorded at the apical septal area. 2. Pacing from the RVA, RVOT resulted in manifest entrainment. Pacing from the left PHA showed concealed entrainment in 7/10 pts. The left bundle branch or left peri-Hisian ventricle (7/10 pts) was captured with post-pacing interval (PPI, 407 짹 70 ms) matching VT cycle length (401 짹 71 ms). Stimulus to QRS intervals (359 짹 62 ms) closely approximated PHA electrogram(egm) to QRS intervals (360 짹 64 ms). 3. His bundle capture (7/10 pts) resulted in manifest entrainment with long postpacing intervals (PPI, 475 짹 80 ms, tachycardia cycle length, 403 짹 75 ms). Spontaneous dissociation of the His bundle egm was recorded without interruption of the tachycardia in 2 pts. 4. The distance between the tachycardia exit region (earliest ventricular activation) and the PHA (where concealed entrainment was achieved) was measured 50 짹 3 mm by CARTO mapping. 5. VT ablation was performed successfully at the LV septal areas with prominent Purkinje potentials (P potential to QRS = 44 짹 24 ms) which was located between the exit and the PHA. CONCLUSIONS Reentry circuit of the idiopathic LV tachycardia may extend from the apical septal to Peri-Hisian area near the proximal His-Purkinje system. Peri-Hisian area appears to be the entry of the protected slow conduction zone constituting the upper turnaround of the reentry circuit.