BACKGROUND AND OBJECTIVES: Premature ventricular contraction (PVC) or ventricular tachycardia (VT) that originates from the aortic cusp (AC) has a similar left bundle branch block (LBBB) pattern with a inferior axis as those LBBB patterns originating from the right ventricular outflow tract, but the electrocardiogram (ECG) characteristics are distinct. We sought to characterize the ECG morphology of PVCs or VTs from the AC and to assess whether these foci exit out to the surrounding 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 mapping at the AC, the anterior interventricular vein (AIV) and the anterior mitral annulus (AMA). The conduction velocities (CV) between the successful ablation site to the epicardium in the AIV, and the endocardial earliest activation (EA) site at the AMA were calculated by triangular algebra at right anterior oblique (RAO) 35degrees and left anterior oblique (LAO) 35degrees, respectively. RESULTS: Successful 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 from the LCC had rS or S waves in lead I and R or RS waves in V1, whereas those from the RCC had R waves in lead I and an rS wave in V1. The CV between the successful 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.