Because syncope may occur intermittently in patients with carotid sinus hypersensitivity, a knowledge of its natural history is needed as a basis for interpreting the usefulness of therapy. Fifty-six consecutive patients are described (47 men and 9 women; mean age 61 years) with carotid sinus hypersensitivity and syncope in whom 24 hour ambulatory monitoring and intracardiac electrophysiologic study revealed no other cause for the syncope. The mean duration of symptoms was 44 months (range 1 to 480) and the mean number of episodes was 4.0 (range 1 to 20). During a follow-up period of 6 to 120 months (median 40), syncope recurred in 3 of 13 patients who received no treatment, in 2 of 23 patients who received a pacemaker and in 4 of 20 patients who received anticholinergic drugs (incidences corrected for totals available at follow-up: 27, 9 and 22%, respectively). Two-thirds of the patients receiving no treatment were asymptomatic compared with all nine of the patients with syncope and a pure cardioinhibitory response to carotid sinus massage who received an atrioventricular (AV) sequential pacemaker. Although pacing was effective in abolishing syncope, its use should be reserved for recurrent episodes because of the high rate of spontaneous remission of symptoms.