The impact of obesity on risk of atrial fibrillation (AF) after coronary artery bypass grafting (CABG) is poorly understood. This study was performed to investigate the relation between body mass index (BMI; kilograms per square meter) or body surface area (BSA; square meters) and AF after CABG. Postoperative AF was assessed in a cohort of 7,027 consecutive patients without preoperative AF undergoing isolated CABG at Baylor University Medical Center from January 1, 1997 to December 31, 2006. Two propensity-adjusted models controlling for risk factors identified by the Society of Thoracic Surgeons and other clinical/nonclinical details were used. After adjustment, BMI and BSA (modeled using smoothing techniques to avoid categorization) were strongly associated (p <0.0001) with postoperative AF. Although evidence existed that gender was associated with AF (p <0.0001 and p = 0.1088 for BSA and BMI models, respectively), there was no indication that the effect of BMI or BSA on postoperative AF varied by gender. In conclusion, this study demonstrates that increased BMI and BSA are associated with a higher risk of AF after CABG and that risk for men is higher for the entire BSA spectrum and for extreme values of BMI.