Mapping in a patient undergoing radiofrequency ablation for a left-sided concealed accessory pathway showed that the site with the shortest VA conduction time was in the great cardiac vein. Epicardial radiofrequency delivery at that site was successful. After ablation, a potential dissociated from both atrial and ventricular activity during sinus rhythm and atrial pacing was noted on the distal bipole of the ablation catheter. During incremental ventricular pacing, intermittent ventricular to potential conduction was observed. This indicates complete block at the atrial-accessory pathway interface and impaired conduction at the ventricular interface. This dissociated activity originating neither from the ventricle nor from the atria provides evidence of automatic accessory pathway activity.