Permanent pacemakers capable of noninvasive electrophysiologic testing were used to study and treat 26 patients with spontaneous sustained ventricular tachycardia (VT) or fibrillation (VF). One hundred nine episodes of sustained VT or VF were induced in these patients. In 8 patients spontaneous VT was reverted by noninvasive means. Drug changes based on noninvasive testing were made in 12 patients. In the 1- to 67-month follow-up period, drug therapy based on noninvasive electrophysiologic testing was predictive of outcome in patients with spontaneous arrhythmias. Thus, noninvasive electrophysiologic testing using permanent pacemakers is a useful method for studying and treating patients with recurrent sustained ventricular arrhythmias.