Background: Although it has been known that atrial fibrillation (AF) is commonly combined with sinus node dysfunction (SND), it is unclear whether new onset AF in patients with SND is hemodynamic effects of pacemaker or disease process itself. Method: We retrospectively analyzed atrial high rate episodes (AHRE) and ECG in 101 patients with SND without previous history of AF who underwent pacemaker implantation (male 29.7%, 64.1짹14.3 years old, AAIR:DDDR=73:28). All the patients have been followed-up at pacemaker clinic biannually for 97.8짹74.3 months. We counted AHRE>2min and defined clinical AF as AHRE>24hr. Results: 1. The incidence of AHRE (35.6% vs. 46.4%, p=NS) or new onset clinical AF (19.2% vs. 17.9%, p=NS) were not different in AAIR vs. DDDR, in spite of significantly higher V pacing % in DDDR (0.0% vs. 30.1%, p<0.0001). 2. In AAIR, the incidence of persistent AF, paroxysmal AF, AHRE < 24hr, no AHRE were 1.4%, 17.8%, 16.4%, and 64.4%, respectively. There was no difference in A pacing % (p=NS). 3. In patients with new onset clinical AF after AAIR implantation, LA size was bigger (47.7짹10.8 mm vs. 40.4짹3.9 mm, p=0.004), follow-up duration was longer (156.4짹60.8 mo vs. 94.3짹73.6 mo, p=0.003), and the incidence of valvular heart disease was higher (28.6% vs. 10.6%, p=0.05) as compared with no AHRE. Conclusion: New onset AF after pacemaker implantation in SND patients was not associated with pacing mode or A/V pacing %, but more common in patients with large LA and longer duration of follow-up. Newly developed AF in patients with SND after pacemaker implantation assumed to be a sequence of progression of atrial disease and aging process rather than consequences from hemodynamic effect from pacing of device.