Introduction: Increased atrial effective refractory period (AERP) dispersion is well correlated with vulnerability to atrial fibrillation (AF). However, the preoperative electrophysiologic characteristics of atrial abnormalities that may play an important role in the development of AF postoperatively in patients with coronary artery bypass grafting (CABG) have not been investigated in detail. Methods and Results: Fifty-six consecutive patients who underwent CABG were enrolled in this study. Eighteen patients (14 men and 4 women; mean age 57.7 ± 5.2 years) with AF in the early postoperative period and 38 patients (28 men and 10 women; mean age 56.3 ± 6.4 years) without AF were compared with regard to preoperative clinical, echocardiographic, angiographic, and electrophysiologic parameters. Preoperative PA interval and AERP dispersion values were higher (P < 0.05) in patients who developed AF in the early postoperative period. PA interval (P < 0.05, odds ratio = 1.64, 95% confidence interval 1.17-2.30), AERP in the high right atrium (AERPHRA; P < 0.05, odds ratio = 0.94, 95% confidence interval 0.91-0.97), AERP in the right posterolateral atrium (AERPRPL; P < 0.05, odds ratio = 0.79, 95% confidence interval 0.63-0.98), AERP in the distal coronary sinus (AERPDCS; P < 0.05, odds ratio = 0.84, 95% confidence interval 0.74-1.02), and AERP dispersion (P < 0.001, odds ratio = 1.29, 95% confidence interval 1.12-1.47) were independently related to post-CABG AF in univariate analysis. Increases in preoperative PA interval and AERP dispersion were found to be associated with a high risk for development of post-CABG AF. Conclusion: AERP dispersion is a suitable electrophysiologic indicator for atrial vulnerability. The presence of increased preoperative AERP dispersion and PA interval may indicate patients at high risk for development of AF in the early postoperative period after coronary artery bypass grafting.