The frequency of inducible tachycardia was assessed in patients presenting with syncope whose noninvasive evaluation did not reveal a cause for syncope. It was also determined whether treatment of tachyarrhythmias during programmed electrical stimulation would prevent recurrence of syncope. One hundred five patients were studied and 97 were followed up for a mean period of 25.8 months. Sixty-eight patients (65%) did not have inducible tachycardia. Sixty of these 68 patients could be followed up; 12 (20%) had recurrent syncope. Ventricular or supraventricular tachycardia was inducible in 37 patients (35%). The frequency of organic heart disease was not higher in this group or in those with inducible ventricular tachycardia as compared with those with inducible supraventricular tachycardia. Three patients with inducible ventricular tachycardia died suddenly or were resuscitated from cardiac arrest, and an additional seven had recurrent syncope; thus, the total recurrence rate was 27%. Of 23 patients undergoing effective therapy as predicted by electrophysiologic testing, 3 (14%) had a recurrent event. Results were significantly different in patients receiving ineffective therapy as judged by electrophysiologic testing. Of 13 patients in this latter category, 7 patients (54%) had recurrence of syncope or cardiac arrest (p less than 0.05). In three patients, recurrence took place a mean of 5 months after cessation of therapy; on resumption of effective therapy, no syncope recurred for 15.6 months (p less than 0.025). Tachycardia is frequently induced in patients with syncope of unknown origin, whether or not organic heart disease is present. Treatment of inducible tachycardia may prevent recurrence of syncope.