Background and Objectives : Premature ventricular contraction (PVC) or ventricular tachycardia (VT) that origi-nates from the aortic cusp (AC) has a similar left bundle branch block (LBBB) patern with a inferior axis as those LB paterns originating from the right ventricular outflow tract, but the electrocardiogram (ECG) characteristics these foci exit out to the surounding epicardium by preferential conduction, resulting in an ECG with epicardial foci. Subjects and Methods : The study subjects were ten patients (M:F= 6:4, 40.9?11.6 years old) with VTs or PVCs that originated from the AC and they underwent radiofrequency catheter ablation (RFCA). We performed simultaneous activation maping at the AC, the anterior interventricular vein (AIV) and the anterior mitral annulus (AMA). The conduction velocities (CV) between the sucesful ablation site to the epicardium in the (EA) site at the AMA were calculated by triangular algebra at right anterior oblique (RAO) 35 and left anterior oblique (LAO) 35, respectively. Results: Sucessful ablation sites were above the left coronary cusp (LCC) in 7 patients, above and beneath the right coronary cusp (RCC) in 1 patient each, respectively, and beneath the LCC in 1 patient. The QRS width was 149.2?19.9 ms, the maximal depolarization time (MDT) was 88.9?14.9 ms and the ratio of the MDT to the QRS was 59.5?5.7%. The PVC lead I and an rS wave in V1. The CV between the sucesful ablation site at the AC to the epicardial EA site (1.7?0.8 m/s) was faster than that to the endocardial EA site (0.8?0.4 m/s, p<0.05). Conclusion : Most of the PVC/VTs from the AC originated from the above LCC and they displayed a faster CV to the epicardial side of the AIV than that to the endocardial side of the AMA. This suggests the existence of preferential conduction from the AC to the left ventricle (LV) epicardium. (Korean Circ J 2007 ;37:616-622)