Electrophysiologic studies were performed in 51 patients with syncopes of unexplained origin. 25 patients (49%) had organic heart disease. Electrophysiologic testing included determination of corrected sinus node recovery time, AV-nodal effective refractory period, AH- and HV-intervals, and AV-nodal Wenckebach rate. During programmed right ventricular stimulation, 1-3 premature stimuli were used. 26 patients (53%) had an abnormal outcome that strongly suggested an arrhythmogenic cause of the reported syncopes. In ten patients (20%), corrected sinus node recovery time was prolonged; AV-nodal conduction disturbance was manifest in two patients (4%); reversibility with atropine was shown in one patient. Six patients (12%) had an infrahisian conduction delay with an HV-interval longer than 70 ms. Eight patients (15.6%) had either symptomatic ventricular tachycardias (n = 4), AV-nodal reentry tachycardias (n = 2), or inducible symptomatic rapid atrial fibrillation (n = 2). In one additional patient, ventricular tachycardias could not be reinitiated after ending tricyclic antidepressant drug medication. The diagnostic yield of the electrophysiologic study was not influenced by the presence of organic heart disease. Patients with prolonged corrected sinus node recovery time, prolonged HV-interval, and irreversible AV-conduction delay underwent pacemaker implantation (n = 17). Patients with rapid response to programmed stimulation received antiarrhythmic medication, the efficacy of which was assessed by serial electrophysiologic testing until non-inducibility was obtained. The mean follow-up period was 11 months (1-31 months). Overall 2-year mortality was 17%. In 4/5 patients, death was unrelated to the cause of syncope.(ABSTRACT TRUNCATED AT 250 WORDS)