Hyperactivity of the carotid sinus reflex is common in older men. However, an unequivocal diagnosis of carotid sinus syncope is difficult to establish because the symptoms are nonspecific, and both hyperactivity of the carotid sinus reflex and syncope are common. Twenty-one men were evaluated for episodes of lightheadedness or syncope, or both, associated with a hypersensitive carotid sinus reflex. Seventeen patients had the cardioinhibitory type, two the vasodepressor type and two both the cardioinhibitory and vasodepressor types. Patients with the cardioinhibitory type benefited from the insertion of a permanent pacemaker if they had multiple episodes of syncope. A history of syncope associated with some event capable of stimulating the carotid sinus was also helpful in selecting patients for pacemaker treatment. The combination of the cardioinhibitory and vasodepressor types may be missed unless carotid sinus stimulation is repeated after the administration of atropine. The results of electrophysiologic studies in 17 patients with the cardioinhibitory type suggest that intrinsic sinus nodal dysfunction is not the major cause for asystole after carotid sinus stimulation.