Background: The majority of idiopathic ventricular arrhythmias (VAs) are known to be originated from right ventricular outflow tract (RVOT) and some of them may originate from the pulmonary artery (PA). We investigated the prevalence, electrocardiographic and electrophysiologic characteristics of idiopathic VAs arising from the PA. Methods and Results: Data obtained from 58 patients (mean age: 48.3짹10.3 yrs, male: 17) who underwent successful catheter ablation of the RVOT VAs were analyzed. Eight of 58 patients (13.8%) have PA origin (PA group) and 50 (86.2%) have infra-valvular origin (infravavular group). There were no differences in QRS morphologies including R-wave amplitude on inferior leads, R/S ratio on lead V2, and precordial transition zone between the PA group and the infravalvular group, except for aVL/aVR ratio of Q-wave amplitude (1.6짹0.5 vs. 1.2짹0.4, P=0.038, respectively). On intracardiac electrograms, atrial potentials were recorded in majority of the PA group and none of the infravalvular group (75% vs. 0%, P<0.001). The amplitude of local ventricular potentials during the VAs was significantly smaller in the PA group than in the infravalvular group (0.6짹0.4 vs. 1.1짹0.8, P=0.026). The earliest activation time preceded the QRS onset during VAs was significantly greater in the PA group than in the infravalvular group (-46.3짹13.6 vs. -36.1짹9.0 ms, P=0.012). Of the PA group, 2 have polymorphic ventricular tachycardia (25%) and 2 have double transition zone on lead V1-2 and V2-3 (25%). Conclusions: Because significant number (13.8%) of idiopathic VAs with QRS morphology of the RVOT has the PA origin, activation mapping inside the PA should be considered, especially in patients with polymorphic VT or double transition on precordial leads.