Background Current practice related to the management of atrial fibrillation (AF) complicating coronary artery bypass grafting (CABG) is uncertain. Methods We examined management of post-CABG AF in the PREVENT-IV trial, and we explored patterns of use of postoperative rhythm versus rate control and anticoagulation for AF by geographic region and type of site. We also compared outcomes of patients who developed post-CABG AF (663) with those who did not (2,131). Results The incidence of AF was 24%. Post-CABG AF was treated with a rhythm control strategy in 81% of patients and with warfarin in 23% of patients. Although there were significant variations across sites in the management of post-CABG AF, patterns of use of postoperative rhythm versus rate control and anticoagulation did not differ by geographic region or by whether or not the enrolling site was an academic institution. Mortality was higher in patients with post-CABG AF than patients without AF at 30 days (1.5% vs 0.7%, P = .01) but not at 3 years (6.9% vs 4.9%, P = .41). There was a trend toward a higher risk of mortality or stroke at 30 days in patients with AF (2.4% vs 1.9%, P = .08). Conclusion Although a rhythm control strategy was used in most of the patients in this trial and the overall rate of use of warfarin was low, the significance of these findings is uncertain because of the lack of data from randomized clinical trials. The substantial variations in the management of post-CABG AF across sites are likely because of definitive data on the most effective therapies, highlighting the need for clinical trials on rate versus rhythm control and on anticoagulation for AF in this setting.