Background: Idiopathic ventricular fibrillation (VF) or polymorphic ventricular tachycardia (PVT) arising from the right ventricular outflow tract (RVOT) is occasionally observed. The difference in the initial ventricular premature contraction (VPC) between VF/PVT and monomorphic VT (MVT) from the RVOT, however, has not yet been fully investigated. Methods and Results: The electrocardiogram findings and the clinical characteristics were compared between 14 patients with PVT and 77 with MVT. The episodes of syncope were more frequent in the VF and/or PVT group (57%) than in the MVT group (10%). An initial VPC with a positive QRS complex in lead I was observed in 10 (71%) of 14 patients with VF/PVT, and in 27 (35%) of 77 patients with MVT (P<0.05). Although radiofrequency (RF) catheter ablation targeting the trigger VPC often produced a morphological change, VF/PVT was eliminated in 13 (93%) of 14 patients after additional RF applications. Conclusions: Malignant arrhythmias from the RVOT, although rare, should be considered when the patient has a syncopal episode and VPC with a positive QRS complex in lead I.  (Circ J 2013; 77: 968–974)