Introduction. Red cell distribution width (RDW) has been associated with poor outcomes in patients with cardiovascular diseases. However, little is known about the role of RDW in prediction of new-onset atrial fibrillation (AF) after coronary artery bypass grafting (CABG). We aimed to investigate the relation between the RDW and postoperative AF in patients undergoing CABG. Methods. A total of 132 patients undergoing nonemergency CABG were included in the study. Patients with previous atrial arrhythmia or requiring concomitant valve surgery were excluded. We retrospectively analyzed 132 consecutive patients (mean age, 60.55 +/- 9.5 years; 99 male and 33 female). The RDW level was determined preoperatively and on postoperative Day 1. Results. Preoperative RDW levels were significantly higher in patients who developed AF than in those who did not (13.9 +/- 1.4 vs. 13.3 +/- 1.2, p = 0.03). There was not any correlation between postoperative RDW levels and AF. Using a cutpoint of 13.45, the preoperative level correlated with the incidence of AF with a sensitivity of 61% and specificity of 60%. Conclusion. Preoperative RDW level predicts new-onset AF after CABG in patients without histories of AF.