BACKGROUND The purpose of this study was to identify the prevalence, preferential sites of tachycardia origin, electrophysiologic characteristics, or efficacy of catheter ablation of idiopathic VAs originating from the specific sites of ventricular endocardium, epicardium, aortic cusp, and near the conduction system. METHODS 316 consecutive patients (mean age 44짹14 years) who underwent catheter ablation for idiopathic VAs were studied. And we studied 36 consecutive patients (11.5%, 42짹 14 years, VT/PVCs: 25/11) who attempted but failed to eliminate VAs with the first catheter ablation (US group). We compared study group with successful ablation group (S group) to assess predictors to affect unsuccessful ablation. RESULTS In 312 patients, 180 RVOT VAs (57.7%) were identified.In 132 non-RVOT VAs, LV endocardium in 35 (26.5%), LVOT in 12, LV epicardium in 9, aortic cusp in 20, RV endocardium in 9 , pulmonary artery in 8, great cardiac vein in 4, papillary muscle area in 11, idiopathic LV fasicular VT(ILVT) in 18, and parahisian area in 6 were observed. The Septum (n=20) and mitral annulus (n=6) were more prevalent site in LV endocardium and LVOT VAs, respectively. In 9 LV epicardial origin, dilated cardiomyopathy was combined in 4 patients (44.4%). Left coronary cusp (n=12) was more prevalent than right coronary cusp (n=4) in aortic cusp VAs. In RV endocardium VAs, prolonged procedure times and higher recurrence rate was observed than RVOT VAs. Papillary muscle VAs showed multiple notches of the R wave in the 3 or more precordial leads and broader QRS width than that of ILVT (156짹16 min vs. 124짹60 min; P=0.003). Parahisian VAs had narrow QRS width than RVOT VAs (114.3짹19.9ms vs 147.0짹15.3ms, P=0.008) and revealed early transition in LV, and late transition in RV whether QRS morphology was RBBB or LBBB. And recurrence rate was higher than other sites.In US group,there was significantly higher portion of VT(55.8%) than that of S group (39.0%, P=0.03). The LV ejection fraction was lower in US group (38.0짹7.2%) than in S group (43.7짹6.9%, P=0.02). The earliest activation time prior to QRS onset in US group (29.8짹7.8ms) was shorter than S group (37.4짹8.4ms, P=0.04). There was a significant difference in VAs origin from RVOT (41.8% in US group vs. 60.2% in S group, P=0.02). CONCLUSIONS The result from catheter ablation of site specific idiopathic VAs could guide more effective and better ablation strategy.A VT as presenting VAs,the severity of LV dysfunction, shorter earliest activation time and non-RVOT origin were associated with unsuccessful ablation outcome, in whom repeated catheter ablation was frequently required.