Objectives: This study investigated the prevalence and electrocardiographic and electrophysiologic characteristics of aortic root ventricular arrhythmias (VAs). Background: Idiopathic VAs originating from the ostium of the left ventricle may be ablated at the base of the aortic cusps. Methods: We studied 265 patients with idiopathic VAs with an inferior QRS-axis morphology. Results: The successful ablation site was within (or below) the aortic cusps in 44 patients (16.6%). The site of the origin was the left coronary cusp (LCC) in 24 (54.5%), the right coronary cusp (RCC) in 14 (31.8%), the noncoronary cusp (NCC) in 1 (2.3%), and at the junction between the LCC and RCC (L-RCC) in 5 (11.4%) cases. The maximum amplitude of the R-wave in the inferior leads was significantly greater with an LCC than with an RCC origin (p < 0.05). The ratio of the R-wave amplitude in leads II and III was significantly greater with an LCC than with an RCC origin (p < 0.01) and was significantly smaller in the NCC than in the other sites (p < 0.0001). The ventricular deflection in the His bundle electrogram was significantly later relative to the surface QRS with an LCC or L-RCC origin than with an RCC or NCC origin (p < 0.0001). The ratio of the atrial-to-ventricular deflection amplitude was significantly greater in the NCC than in the other sites (p < 0.0001). No other factors predicted the site of origin. Conclusions: Idiopathic VAs are more common in the LCC than in the RCC and rarely arise from the NCC. The electrocardiogram is useful for differentiating the site of origin. © 2008 American College of Cardiology Foundation.