OBJECTIVES: Right atrial septal (RAS) pacing can shorten global atrial activation time, resulting in significant reduction of recurrence of atrial fibrillation (AF) in patients with sick sinus syndrome (SSS). This study was to compare P-wave morphology and duration in pacing from the RAS wall and the right atrial appendage (RAA), and to compare AF event rate between pacing at RAS and RAA. METHODS: 26 patients (M:F=10:16, 67.0짹11.2 yrs) who underwent pacemaker implantation due to SSS (n=20) and complete AV block (n=6) were enrolled. Chronic AF patients were excluded. Patients were divided into RAS pacing (n=12) and RAA pacing (n=14). For RAS pacing, an active fixation atrial lead was positioned at the coronary sinus ostium, or lower edge of fossa ovalis (n=10), or Bachmann���s bundle (BB, n=2). Patients were discharged at a pacing rate of 65 bpm after setting of the optimal AV delay. The electrocardiograms (ECGs) of patients with RAS pacing were compared with those of patients with RAA pacing, and compared AF incidence (lasted > 30 minutes) in each pacing group during follow-up. RESULTS: In lead V1 and II, the P wave duration of RAS pacing was significantly shorter than RAA pacing (V1: 72짹20 ms vs. 143짹26 ms, p<0.01, lead II: 73짹16 ms vs. 124짹35 ms, p<0.01). In the frontal plane, patients with RAS pacing showed a superior P-wave axis between -47.5짹63.3 and 2.5짹68.2 degrees. P-wave axis of RAA pacing was inferior between 36.4짹48.8 and 79.3짹40.0 degrees. The terminal part of biphasic P waves in lead V1 in RAS pacing was positive (negative-positive) or flat, in contrast, negative in all patients with RAA pacing (positive-negative) except one. During follow-up of 7짹5 months, AF occurrence rate in RAS pacing (16.7%) was lower than that in RAA pacing (35.7%). CONCLUSION: The total atrial activation time during RAS pacing is shorter than that during RAA pacing. The P wave axis in RAS pacing and RAA pacing are significantly different. The terminal positive part of biphasic P waves in lead V1 during RAS pacing indicates that atrial activation occurs firstly in the left atrium and followed by the right atrium. This atrial activation pattern by RAS pacing was beneficial to prevent new-onset of AF.