It has been demonstrated that atrial fibrillation (AF) frequently develops in pacemaker (PM) recipients without arrhythmia history. It remains to be determined whether newly developed AF increases risk of stroke and therefore these AF patients are under optimal anticoagulation. We sought to determine the incidence and predictors of new onset of AF [AF defined as an atrial high rate episode (AHRE)���2 minutes of stored electrograms竊�in patients following PM implantation and to define the status of therapy for AF including anticoagulation. Methods and Results: We evaluated 143 patients (49 male; age 66 짹 16 years) without documented AF who underwent PM implantation for sinus node dysfunction (SND, n = 51) or atrioventricular block (AVB, n = 92). Within 1.9 year of PM implantation, an AHRE���2 minutes was detected in 39 (27.2%) patients. Of these, 27 (77%) patients had an AHRE���1 hour, 5 (13%) patients had���1 day, 3 (8%) patients had���1 week, and 4 (10%) patients had persistent AF���7 days. Mean AHRE episodes were 149 per month. Pacing mode was DDD/DDDR in 82% and AAIR in 18%, in patients with SND (41%) and AVB (59%), respectively. Cumulative ventricular pacing (82짹30%) was significantly higher than those without AHRE (66짹43%)(P<0.05). 28 (72%) of AHRE was asymptomatic and symptoms did not match with an actual AHRE in most patients. The number of patients who are taking warfarin was only 3 (8%), aspirin 17 (44%) and ticlopidone 4 (10%). And only 12 (31%) patients are taking antiarrythmic drugs. Conclusions. Within 1.9 year of PM implantation in patients with SND and AVB without history of AF, AF is detected in 27% of patients. Higher cumulative ventricular pacing is associated with new onset of AF. Most are asymptomatic and are not adequately treated to prevent from stroke.