Background: For radiofrequency(RF) ablation of idiopathic left ventricular tachycardia(ILVT), termination of tachycardia with RF energy delivery is considered to be a hallmark of successful energy delivery. However, in patients with non-inducible-sustained VT for mapping, the evaluation of procedural success and ILVT recurrences can be problematic. We have observed thermal responses of patients with ILVT during RF delivery. We therefore describe thermal mapping herein to guide the ablation of ILVT. Methods & Results: The study subjects were 10 ILVT patients (9 males : 1 female, mean age 33.5  7.5 years). RF energy was delivered to the patients during sinus rhythm because of repeated mechanical termination of VT by catheter manipulation or difficulty with re-induction of tachycardia. During each energy delivery, we focused on the rhythm changes, especially the occurrence of repetitive ventricular responses, such as venricular premature contraction (VPC), non-sustained VT, or VT having a similar morphology to clinical VT. The incidence of the repetitive ventricular responses was examined and the clinical implication was assessed based on the clinical course. The attempted RF ablation site was selected to show Purkinje-potential and/or where the tachycardia was presumed to be mechanically-terminated by catheter movement. The rhythm response varied according to each ablation site, from no change in rhythm to induction of ventricular fibrillation. However, the repetitive ventricular responses were noted at least more than once in all patients. The mean number of RF applications was 10.6  4.5/patient. There was 1 patient who received a second procedure due to recurrence of ILVT 3 days after the 1st procedure: no repetitive ventricular responses had been observed during the 1st procedure. Nevertheless the 1st procedure was regarded to be successful since there was no re-induction of VT. Conclusions: Based on the favorable clinical outcomes of the study subjects, ablation-induced repetitive ventricular responses, especially showing a similar QRS morphology to clinical VT, may be a useful marker for selecting an ablation site in ILVT.