Since July 1982, 12 patients have undergone catheter ablation for refractory arrhythmias. The ablated pathway was the His bundle in 8 patients, an accessory pathway in 3 patients and a ventricular reentrant circuit in 1 patient. The 8 patients in whom the His bundle was ablated comprised 3 cases of atrial flutter, 3 cases of atrial fibrillation, 1 case of atrio-ventricular tachycardia and 1 case of junctional reentrant tachycardia. Results were complete heart block in 6 of these cases, right bundle branch block and first degree a-v block in 1 case and first degree a-v block in the last case. A pacemaker was implanted in 7 patients. The total amount of energy used varied from 50 to 400 joules. In none of these 3 patients did tachyarrhythmia recur. The 3 cases with ablation of the accessory pathway comprised one success, one partial success and one failure. The energy used was 60 to 100 joules. The patient treated for ventricular tachycardia has had no new episodes of tachycardia 4 months after the procedure. The energy used was 200 joules. The mean follow-up for the 12 patients is 12 +/- 7 months. Apart from 2 episodes of thrombophlebitis, no serious complication was noted. Catheter ablation technique is a new and effective treatment for refractory supraventricular tachyarrhythmias. Concerning ventricular arrhythmias, our experience is not sufficient to allow conclusions to be drawn.