Background: Atrial fibrillation (AF) after coronary artery bypass graft surgery (CABG) constitutes the most common sustained arrhythmia and results in prolonged hospitalization. The purpose of this study was to assess simultaneous right and left atrial pacing as prophylaxis for postoperative atrial fibrillation. Methods and Results: From July 2003 to May 2004, 120 patients without structural heart disease and who underwent CABG were randomly classified into one of the following 3 groups: biatrial pacing (BAP), left atrial pacing (LAP), and no pacing (control). Atrial pacing was performed for 4 days. Post-CABG AF was significantly reduced in BAP group compared to single-site and control group (BAP, 17.5%; LAP, 30%; control, 45%; p=0.02). The mean length of hospital stay was significantly reduced in BAP group. Hospital charges were not significantly different between three groups. The mean length of hospital stay was most significantly reduced in BAP group (6.1±1.2 versus 9.0±4.1 days in the control groups; p=0.002, and 8.7±1.3 days in LAP groups; p=0.01). The mean length of stay in the intensive care unit was also significantly reduced in the BAP group (2.8±0.7 versus 4.6±4.5 days in control group; p=0.04, and 4.2±3.2 days in LAP group; p=0.01). Conclusions: Simultaneous right and left atrial pacing is well tolerated and is more effective in preventing post-CABG AF than single-site pacing, and, results in a shortened hospital stay. Identifying patients at risk for developing postoperative AF and using this prophylactic method may be the optimal effective strategy.