Tachyarrhythmias mediated by an accessory atrioventricular pathway and which are refractory to drug therapy have been treated surgically with variable success. Early results of direct-current catheter ablation were encouraging but were associated with complications such as barotrauma and the need for a general anaesthetic. We have investigated the endocardial application of radiofrequency current which is a potentially safer technique. Of 105 patients with an accessory atrioventricular pathway, 79 were located on the left side of the heart and 32 on the right side. Accessory pathway conduction was permanently abolished in 93 (89%) patients. Complications developed in 3 patients: thrombotic occlusion of a femoral artery, arteriovenous fistula formation at the site of groin puncture, and left ventricular rupture with cardiac tamponade after direct-current shocks. There were no deaths from the procedure. We conclude that radiofrequency current catheter ablation is both effective and safe for patients with symptomatic tachyarrhythmias mediated by accessory atrioventricular pathways.