To assess the incidence and clinical characteristics of carotid sinus hypersensitivity and the relationship to electrophysiologic findings, 76 patients with unexplained syncope underwent carotid sinus massage during electrophysiologic studies for syncope evaluation. Twenty-one patients (28%) were found to have carotid sinus hypersensitivity. Of these 21 patients, 11 (52%) had coronary artery disease, two (10%) had hypertensive heart disease, and eight (38%) had no organic heart disease. During electrophysiologic studies, abnormal sinus node function was found in three patients (14%), abnormal atrioventricular (AV) node function was noted in four (19%), and combined abnormal sinus node and AV node functions were seen in three (14%). Eleven patients (53%) had a normal electrophysiologic study. During carotid sinus massage, sinus arrest alone was observed in 12 patients (57%), and combined sinus arrest and AV nodal block was seen in nine (43%). Thirteen patients were treated with a permanent pacemaker, in whom either carotid sinus massage reproduced the symptom or concomitant sinus node or AV node abnormality, or organic heart disease was present. With a mean follow-up of 42 +/- 19 months, none of these 13 patients had recurrent syncope. However, one of eight patients (13%) who did not receive a pacemaker had recurrence of syncope. Subsequently, this patient has done well after implantation of a pacemaker. These observations suggest that there is a significant incidence of carotid sinus hypersensitivity in patients with unexplained syncope. Permanent pacing appears to be beneficial in selected patients based on clinical and electrophysiologic findings.