Twenty-two patients with refractory supraventricular arrhythmias were treated by catheter-delivered high-energy shocks to the atrioventricular conduction system. All patients had a minimum follow-up period of six months (mean +/- SD, 15 +/- 9 months), at which time 21 of the 22 patients were free of symptoms and required no antiarrhythmic therapy. Permanent pacemakers were implanted in all patients. These results show that transvenous ablation or modification of atrioventricular conduction is a safe and effective technique to treat a wide range of supraventricular arrhythmias, and obviates the need for open-heart surgery for the interruption of atrioventricular nodal conduction.