Abstract Background Postoperative atrial fibrillation (POAF), a common complication following coronary artery bypass graft (CABG) surgery, is associated with increased morbidity and mortality. Inflammation seems to be an important pathogenic factor and elevated preoperative levels of C-reactive protein (CRP) are associated with POAF. However, the association between postoperative CRP and POAF is less established. Purpose To examine the association between postoperative levels of CRP and POAF after CABG. Methods Patients undergoing first-time isolated CABG (Jan 01, 2000 to Dec 31, 2016) were identified using Danish nationwide administrative registries. Patients with CRP measurements from the fourth postoperative day and no history of atrial fibrillation were included. POAF was defined as a postoperative atrial fibrillation rhythm that required medical therapy or cardioversion, with patients being followed for POAF until discharge. The study population was divided into quartiles based on CRP levels. A Cochran-Armitage trend test was used to examine the trend of increased levels of CRP and the associated risk of POAF. The association between CRP levels and POAF was investigated using a multivariable logistic regression analysis adjusted for other patient characteristics. Results The study included 6,711 patients with mean CRP values from the lowest to the highest CRP groups being 66, 109, 150 and 228 mg/L, respectively. Patients in the highest CRP group were older and more likely to be male compared with the lowest CRP group (median age 67 years [P25-P75: 61–73] and 84.7% men vs. median age 64 years [P25-P75: 56–70] and 77.9% men). In the highest CRP group, 35% of patients developed POAF vs. 25% in the lowest CRP group. A dose-response relationship was seen between increasing levels of CRP and the associated risk of POAF (p<0.0001 for trend). An adjusted analysis showed a statistically significant association between the highest CPR group and POAF (OR: 1.44; 95% CI: 1.24–1.69) compared with the lowest CRP group (figure 1). Conclusion High postoperative CRP levels after CABG were associated with POAF development. Future studies may examine whether CRP also predicts worse outcomes and whether these factors could be modified in the development of POAF. Acknowledgement/Funding Internal grant from Copenhagen University Hospital