Background It has been known that atrial fibrillation(AF) commonly occurs in patients with pacemaker. We investigated whether the patients with certain clinical and electrophysiological parameters, or serological markers are more vulnerable to the occurrence of AF with long-term pacing. Methods and Results 76 patients(M:F=27:49, 67.6짹14.3years old, 28 sinus node dysfunction(SND), 48 complete AV block(AVB)) without a previous history of AF who were implanted pacemaker were included, and divided according to the maximum durations of recorded atrial high rate episodes(AHRE): no AHRE, transient AF(2minutes to 24hours), paroxysmal AF(PAF;24hours to 1week), and persistent AF(PeAF;>1week). Echocardiographic findings, medications, proportions of pacing, and serologic markers related matrix remodeling, fibrosis, and chemotaxis(Matrix metalloproteinase(MMP)-9, tissue inhibitor of metalloproteinase(TIMP)-1, transforming growth factor(TGF)-棺, pro-atrial natriuretic peptide(ANP), stromal cell derived factor(SDF)-1慣), were compared. The serum was taken at the time of pacemaker follow up clinic. Results 1.During 56.4짹47.5 months of follow up, 7 patients(9.2%) developed new onset AF(5 PAF and 2 PeAF) and transient AF were documented in 22.4%(17/76) of the patients. 2.The patients with newly developed clinical AF showed higher proportion of SND(71.4% vs.33.3%, p<0.05), higher prescription rate of anti-arrhythmic drugs(57.1짹53.5% vs.11.6짹32.3%, p<0.001), smaller left ventricular diastolic dimensions(47.2짹8.3mm vs.51.6짹5.1mm, p<0.05), and higher serum level of pro-ANP(7.60짹2.63nmol/L vs.3.88짹2.43nmol/L, p<0.001) than those without it. 3.The patients with transient AF had higher proportion of atrial pacing(52.5짹28.7% vs.37.5짹31.8%, p<0.05) and lower level of SDF-1慣(2.65짹0.53ng/mL vs.3.30짹1.03ng/mL, p<0.01) compared with those without AHRE. Conclusion New onset AF was more commonly developed in patients with SND, higher pro-ANP or lower SDF-1慣 level after pacemaker implantation, despite of higher incidence of atrial pacing and taking antiarrhtyhmic drugs. Atrial pressure or volume overload and/or electrical heterogeneity due to long term pacing may contribute the development of AF in patients with pacemaker.