The case of a patient with a symptomatic Wolff-Parkinson-White syndrome undergoing attempted radiofrequency catheter ablation of a posteroseptal accessory pathway is described. Coronary sinus venography revealed the presence of a diverticulum attaching near the os. The electrogram recorded from a catheter placed in the narrow neck of the diverticulum revealed a short atrioventricular time during sinus rhythm. The pathway was easily ablated using radiofrequency energy applied in the neck of the diverticulum, after multiple failed attempts at catheter ablation from the endocardial surface of the posteroseptal space. Our report emphasizes the importance of searching for a coronary venous diverticulum in all patients with posterior accessory pathways undergoing catheter ablation.