A 68-year-old man with symptomatic idiopathic premature ventricular contractions (PVCs) underwent electrophysiological testing. Radiofrequency catheter ablation was unsuccessful at the earliest endocardial ventricular activation site in the left coronary cusp. Epicardial mapping via the cardiac veins was then performed. Balloon-occluded coronary sinus venography revealed the small branches of the anterior interventricular vein. Mapping with a microcatheter revealed the earliest ventricular activation and perfect pace map at the distal portion of the septal perforating branch, suggesting an intramural ventricular septal PVC origin. Catheter ablation was abandoned because of the inaccessibility of the ablation catheter to that site via the venous system. (PACE 2009; e1-e3). © 2009 Wiley Periodicals, Inc.