The natural history of patients with severe carotid sinus syndrome, and the efficacy of permanent pacemaker treatment are not clearly known. A randomized treatment/nontreatment prospective study was performed in 60 patients affected by carotid sinus syndrome whose symptoms were judged to involve risk of major trauma or interfered with their daily activity. They were randomly assigned to 2 groups: 28 patients to no therapy (nonpacing group), and 32 to VVI (n = 18) or DDD (n = 14) pacemaker implant (pacing group). Syncope recurred in 16 patients (57%) of the nopacing group (mean follow-up 36 +/- 10 months) and in only 3 (9%) of the pacing group (mean follow-up 34 +/- 10 months) (p = 0.0002); moreover, 19 (68%) in the nonpacing group needed a secondary pacemaker implant because of the severity of symptoms. The actuarial rate of absence of syncopal recurrence after 1, 2, 3 and 4 years was 64, 54, 38 and 38%, respectively, for the nonpacing group, and 100, 97, 93 and 84%, respectively, for the pacing group (p = 0.0001). The actuarial rate of absence of minor symptoms after 1, 2, 3 and 4 years was 21, 14, 7 and 7%, respectively, for the nonpacing group and 66, 43, 27 and 27%, respectively, for the pacing group (p = 0.002). Reproducibility of carotid sinus reflex was tested after 15 +/- 8 months in 54 patients; an abnormal response to carotid sinus massage persisted in all 54. In conclusion, symptoms recur in most patients with untreated carotid sinus syndrome, and pacing is a useful therapy for preventing recurrences.