Background: The AAI mode is thought to be more physiological than DDD mode, and the preservation of normal intraventricular conduction without ventricular pacing in AAI is thought to impart better ventricular function. However, it remained controversial which pacing mode, AAI or DDD, is more favourable for patients with SSS and normal AV conduction. To evaluate the clinical benefits of AAI pacing in patinents with sinus-node dysfunction (SND) and normal atrioventricular (AV) conduction, we compared two groups of patients, AAI and DDD. Methods: One hundred eighty-six patients (male 86, mean age 58 짹 15 years) without atrial fibrillation were evaluated retrospectively. Pacing mode was AAI in 73 (39%) and DDD in 113 (61%) patients. Mean follow-up duration was 75months. The primary end point was death from cardiac cause or any cause. Secondary end points included the death, hospitalization for stroke or heart failure (CHF), and atrial fibrillation. Results: The incidence of the primary end point did not differ significantly between AAI group (2.8%) and DDD group (7.1%, p=0.46). In patients assigned to AAI pacing, the risk of atrial fibrillation was lower (harzard ratio, 0.84; 95% confidence interval, 0.72 to 0.97, p=0.02). The incidence of hospitalization for CHF was significantly lower in AAI group (0%) than DDD group (8.8%, p=0.03). However, risk of hospitalization for CHF did not differ in multivariate analysis (harzard ratio, 0.27; 95% confidence interval, 0 to 0.01, p=0.95). There was no diffenrence in the incidence of stroke between two groups. The risk of secondary event was lower in patients assigned to AAI pacing (harzard ratio, 0.89; 95% confidence interval, 0.83 to 0.96, p=0.001). During follow-up, four AAI devices (5%) were switched to DDD due to high-degree AV block. Conclusions: AAI pacing, a simple system using a single lead and single-chamber pacemaker, can achieve a better clinical outcome in occurrence of atrial fibrillation and secondary end point. However, some patients with AAI needed to switch to DDD due to high-degree AV block during follow up.