Context Atrial fibrillation is the most frequent arrhythmia following cardiac surgery. Many risk factors for this problem have been studied. Aims The objective was to investigate the relation between perioperative hyperglycemia and postoperative atrial fibrillation (POAF) after coronary artery bypass grafting. Settings and design The study was a retrospective observational study that took place at Benha University Hospital, which is a tertiary referral center. Patients and methods The study was conducted on 100 patients who were admitted for coronary artery bypass grafting. Patients were divided into two groups: group A included 50 patients who developed POAF and group B included 50 patients who did not. Statistical analysis Data were imported into Statistical Package for the Social Sciences (SPSS version 20.0) software for analysis. Qualitative data represented as number and percentage and tested by the χ2 -test. Quantitative data were represented by mean±SD and tested by t-test or Mann–Whitney. Results The authors have found that a history of diabetes mellitus, mean postoperative blood sugar (BS), and maximum postoperative BS levels were more significant (P