Ventricular tachycardia (VT) without structural heart disease or any identifiable predisposing causes for arrhythmia is an uncommon but well-recognized clinical entity. The purpose of this study is to assess the results of catheter ablation therapy and the long-term outcome of patients with idiopathic monomorphic VT in a large patient group. Sixty-one consecutive patients (male/female=40/21; mean age 38+/-16 years) with idiopathic VT underwent electrophysiologic study and an attempt of catheter ablation therapy. The 'left VT' group included 31 patients with QRS morphology of right bundle branch block during VT suggestive of the VT originating from the left ventricle (LV), and the 'right VT' group consisted of 30 patients with QRS morphology of left bundle branch block with normal or right frontal axis deviation suggestive of VT arising from right ventricular outflow tract (RVOT). Idiopathic left VT has sustained VT during the clinical attacks, baseline electrophysiologic study or after isoproterenol infusion; it can be entrained by overdrive ventricular pacing, terminated by verapamil, but not by adenosine (except one case with VT focus at left ventricular free wall). Catheter ablation was successful in 22 (84%) of 26 patients, with recurrence rate of 9%. The successful ablation sites were located at LV inferior-apical septum (16 patients), mid-septum (three patients), high septum (two patients) and high anterior wall (one patient). In the right VT group, 20 (67%) of 30 patients presented clinically repetitive monomorphic VT. Most of the idiopathic right VT (22/30) required isoproterenol to facilitate induction of VT, and were sensitive to both verapamil and adenosine. Successful catheter ablation was achieved in 21 (84%) of 25 patients, with recurrence rate 19%. The successful ablation sites were located at RVOT-septum in 18 patients, and RVOT-free wall in three patients. During a mean follow-up period of 29.2+/-21.7 months (range 1-76 months) after hospital discharge, all patients were alive but one left VT case died of non-cardiovascular cause. We concluded that idiopathic left side and right side VTs have their distinct clinical, electrophysiologic and electropharmacological characteristics suggestive of different underlying mechanisms, and both have a benign prognosis. Furthermore, catheter ablation can be effective in eliminating idiopathic VT originating from the right ventricular outflow tract and left ventricle.