Eight patients with a posteroseptal accessory pathway and symptomatic atrial fibrillation and/or orthodromic reciprocating tachycardia underwent attempted transcatheter ablation of the accessory pathway. A quadripolar electrode catheter was positioned within the coronary sinus such that the proximal pair of electrodes straddled the os. This proximal pair of electrodes was made electrically common and connected to the cathodal output of a defibrillator. A patch electrode placed over the midthoracic spine was connected to the anodal sink of the defibrillator. Two to three transcatheter shocks were delivered, with a cumulative energy of 600 to 900 J. Immediately after the shocks were delivered, retrograde accessory pathway conduction was absent in each patient. Anterograde conduction through the posteroseptal accessory pathway was absent in six patients and could not be assessed in two patients who each had a second right-sided accessory pathway. Long-term results were assessed in seven patient 4 to 11 months after delivery of the shocks. Anterograde and retrograde conduction through the posteroseptal accessory pathway was absent in five patients. In one patient, retrograde accessory pathway conduction was absent and anterograde conduction was present but was slower than at baseline. In this patient, orthodromic tachycardia was no longer inducible and the ventricular rate during induced atrial fibrillation was 150 beats/min, compared with 220 beats/min before the attempted ablation. He has remained asymptomatic without antiaarhythmic drug therapy for 18 months. In one patient, the transcatheter shocks had no long-term effect on accessory pathway conduction. The shocks delivered at the os of the coronary sinus were well tolerated. There was no long-term effect on anterograde conduction through the atrioventricular junction. Retrograde conduction through the atrioventricular junction was impaired in three patients. In four patients who underwent direct or angiographic visualization of the coronary sinus, no abnormalities were seen. Transcatheter ablation of posteroseptal accessory pathways has a long-term success of at least 75% and a low morbidity rate. Closed-chest catheter ablation of posteroseptal accessory pathways is an attractive alternative to surgical ablation to these pathways.