Background: Dual-chamber pacing preserves AV synchrony and may reduce heart failure compared with ventricular pacing in sick sinus syndrome(SSS). However, ventricular desynchronization imposed by ventricular pacing (Vp) was suggested to be associated with increased risk of heart failure (HF). In Korea, there has been no reported data proving this suggestion. Therefore, we investigated the effect of Vp in SSS patients with pacemaker therapy and normal QRS duration (QRS duration < 120ms ). Methods: Between 1990 and 2004, 155 patients (67 male, mean age; 59짹15 years) with SSS and normal QRSduration (QRSd) had undergone ventricular (n= 38) or dual chamber (n= 117) pacemaker implantation for SSS. Cumulative percent ventricular pacing (Cum%Vp) was determined from stored pacemaker data. Clinical outcomes including total death, cardiac death, and HF hospitalization were compared between lower (Cum%Vp��� 80%, n=98, 63.2%) and higher (Cum%Vp > 80%, n=57, 36.8%) Vp group. Results: In total patients, 4 (2.5%) deaths occurred (higher Vp;n=3, 5.9%, lower Vp;n=1, 1.3%). One patients in higher Vp group died with heart failure. 5- year survival was 89.2% in higher Vp group and 100% in lower Vp goup (p=0.03). Heart failure occurred in 20 patients (higher Vp;n=10, 10.2%, lower Vp;n=10, 17.5%). 5- year HF hospitalization free survival was 82.2% in higher Vp group and 91.5% in lower Vp group (p=0.48). Conclusion: There was a tendency of more death and HF hospitalization in higher Vp group. Cum Vp>80% was associated with increase risk of death during pacemaker therapy for SSS in normal baseline QRS duration.