Introduction: Cardiac arrhythmias are the most common complication in cardiac surgery and they play a major role in postoperative mortality and morbidity. New-onset of atrial fibrillation is the most common arrhythmic complication after coronary artery bypass graft surgery and it may result in haemodynamic compromise, thrombotic events, anxiety, and thus a prolonged hospitalization. Materials and methods: A prospective study was performed in the Institute of Cardiovascular Diseases and Transplantation of Targu Mures, Romania. We collected data between January and June 2017. For statistical analysis T-test and Fisher-test (GraphPad Prism 6.0 software, GraphPad Software, Inc, La Jolla, CA 92037 USA) were used. Significance level was set at p<0.005. Results: In the arrhythmic group, the mean age was higher than in the control group. The ejection fraction was lower in the arrhythmic group. In the arrhythmic group, more cases needed higher doses of inotropic drugs than in the control group. The arrhythmic group presented statistically significant hemodynamic and renal postoperative complications. Discussions: Cardiac dilatation, myocardial atrophy, decrease of conduction tissue and atrial fibrosis caused by aging may be responsible for the increased risk of atrial fibrillation after CABG. Echography findings that suggested the increased risk of atrial fibrillation in the postoperative period was the enlarged atrium. The blood supply to the right atrium, the sinoatrial node and the atrioventricular node is mainly conveyed via the right coronary artery, thus the right coronary artery stenosis may be a predictor. The statistically significant hemodynamic and renal postoperative complications in our study are among the most common postoperative complications described in the literature. Conclusions: Postoperative atrial fibrillation is more frequent in the elderly patients (above 60 years) with increased left atrium. Other disturbances that predispose to this arrhythmia are hypoxia and metabolic acidosis.