A multicenter, randomized clinical trial. All patients received a dual chamber pacemaker capable of either single or dual chamber rate modulated pacing. Patients were then randomized to either the single chamber mode or the dual chamber mode. Patients with prior stroke were pre-stratified. Clinical and electrocardiographic data were collected during a 1.5 to 4.5 year follow-up. The primary endpoint was either: first occurrence of stroke, or; total (all-cause) mortality. Secondary endpoints included health status, cost-effectiveness, cardiovascular mortality, composite of any of the three major adverse effects expected in sick sinus syndrome patients (total mortality alone or first stroke or congestive heart failure hospitalization), first occurrence of atrial fibrillation, heart failure score, pacemaker syndrome, health status in women and in the elderly, and outcome of patients with risk factors for pacemaker syndrome. Enrollment was completed in October, 1999 with a total of 2,010 patients.