The purpose of this study was to identify the post-cardiac surgery delirium risk factors and to evaluate clinical outcomes. Data on 90 patients with postoperative delirium after cardiac surgery on cardiopulmonary bypass (CPB) were analyzed retrospectively. The patients were divided into two groups by evaluating the severity of the delirium: light and moderate delirium group (n=74) and severe delirium group (n=16). We found that the rate of early post-cardiac surgery delirium was low (4.17%). We have determined that post-cardiac surgery delirium prolonged the length of stay in the Intensive Care Unit (ICU) by (8.4 (8.6)) and the hospital stay by (23.6 (13.0)) days. The patients had higher preoperative risk scores, their age was 71.5 (8.9) years, the body mass index was 28.8 (4.4) kg/m2, the majority were male (72.2%), and the left ventricular ejection fraction was 46.1(11.9) %. Statistical analysis by multivariable logistic regression has indicated that increasing the dose of fentanyl administered during surgery over 1.4 mg also increased the possibility of developing a severe delirium (OR=29.4, CI 4.1-210.3) and longer aortic clamping time could be independently associated with severe postoperative delirium (OR=8.0, CI 1.7-37.2). After surgery, new atrial fibrillation (AF) episodes amounted to 53.3% and, after distinguishing the delirium severity groups, AF developed in the patients belonging to the severe delirium groups statistically significantly more frequently, 81.8 vs 47.3, where p=0.01. Our data suggest that early post-cardiac surgery delirium is not a common complication, but it prolonged the length of stay at the ICU and in the hospital. The delirium risk factors, such as longer aortic clamping time, the dose of fentanyl and new atrial fibrillation episodes occurring after cardiac surgery, are associated statistically significantly with the development of severe post-cardiac surgery delirium.