A large population of sick sinus syndrome (SSS) patients was analyzed to determine whether age of patients, presence of conduction disturbances and mode of permanent pacing are related to the occurrence of supraventricular tachyarrhythmias, cerebral embolism and cardiac mortality. Three hundred thirty-nine patients permanently paced (135 AAI, 79 DDD, 125 VVI) because of SSS were followed for a mean period of 5 years (range 2 to 10). Patients were divided into 4 groups according to age (less than 70 or greater than 70 years) and the presence or absence of an associated conduction disturbance. Sixty-eight percent of VVI, 55% of AAI and 40.5% of DDD patients were greater than 70 years of age. In the VVI and DDD groups a conduction disturbance was present in 67 of 204 (33%) patients; conduction disturbances were more common in patients greater than 70 years old (46 of 111, 41%) than in those less than 70 years old (21 of 93,22%). The Wenckebach threshold (greater than 140 beats/min) remained unchanged during the follow-up period in 82% of AAI patients. In 9% of these patients, the Wenckebach threshold showed some degree of deterioration, but only in 2 patients was it less than 100 beats/min (1.5%). Spontaneous second-degree atrioventricular block was observed in 7 patients (5%); it disappeared in 6 of these patients when drug therapy was discontinued.(ABSTRACT TRUNCATED AT 250 WORDS)