New-onset atrial fibrillation (AF) after coronary artery bypass grafting (CABG) remains a prevalent problem. We investigated the relationship between sleep apnea and new-onset post-CABG AF during inhospital stay.We prospectively recruited 171 patients listed for an elective CABG for an overnight sleep study. Sleep apnea was defined as apnea-hypopnea index greater than or equal to 5.Among the 160 patients who completed the study, those in the sleep apnea group (n=128; 80%) had larger left atrial diameter (40.4±5.4 vs 38.4±6.0 mm; P=.03) and left ventricular end-diastolic dimension (52.6±7.9 vs 49.2±6.8 mm; P=.03) than those in the non-sleep apnea group. The incidence of new-onset post-CABG AF was higher for the sleep apnea than non-sleep apnea groups (24.8% vs 9.7%; P=.07). There was 1 inhospital death and 2 patients with acute renal failure requiring dialysis after CABG in the sleep apnea group. None of the patients developed inhospital stroke. Multiple logistic regression analysis showed that sleep apnea was an independent predictor of post-CABG AF (odds ratio, 4.4; 95% confidence interval, 1.1-18.1; P=.04).Sleep apnea is prevalent in patients undergoing CABG. It increases the susceptibility to new-onset AF after CABG, probably related to atrial and ventricular remodeling.