We conducted this study to verify the efficacy of ventricular unipolar potential (V-uni) for ablation of idiopathic non-reentrant ventricular tachycardia (idio-VT). The morphology of V-uni at the successful and unsuccessful sites was analyzed in 27 patients with idio-VT [20 with right ventricular outflow tachycardia (RVOVT) and 7 with left ventricular outflow tachycardia (LVOVT)]. The usefulness of V-uni was compared with a pacemapping method and the V-QRS interval. The incidence of QS-pattern V-uni at the successful and best unsuccessful sites were 100 versus 25% (P = 0.000005) in RVOVT and 86 versus 29% (P = 0.10) in LVOVT. The pacemapping scores at the successful and best unsuccessful sites were 11.5/12 versus 11.2/12; NS in RVOVT, and 11.2/12 versus 11.1/12; NS in LVOVT. The mean V-QRS interval at the successful and the best unsuccessful sites were 22.5 +/- 3.8 versus 21.6 +/- 3.4 msec; NS in RVOVT, 15.1 +/- 3.2 versus 12.5 +/- 3.3 msec; NS in LVOVT. The sensitivity (sen) and specificity (spe) of QS-pattern V-uni to determine the optimum target sites were 1.0 and 0.89 in RVOVT and 0.86 and 0.83 in LVOVT, respectively. In the ablation of idio-VT, QS-pattern V-uni is simply and visually identifiable, is very useful, and should be given a high priority when determining the optimum target site.