A 74-year-old man with symptomatic premature ventricular contractions (PVCs) with a right bundle branch block and right inferior axis QRS morphology underwent electrophysiologic testing. During the PVCs, coronary sinus mapping revealed ventricular prepotentials with the earliest activation in the distal great cardiac vein (GCV) where the local ventricular electrograms were smaller in amplitude than the atrial electrograms. The transaortic approach could not reach the earliest activation site within the GCV, but the transseptal catheter ablation successfully eliminated the PVCs on the mitral valve. With catheter ablation of ventricular arrhythmias with pure mitral annular origins, a transseptal approach may be necessary. ©2009 Wiley Periodicals, Inc.