PURPOSE: The purpose of the present study was to investigate the association between the perioperative neutrophil to lymphocyte ratio (NLR) and cardiac surgery patient outcomes. PATIENTS AND METHODS: A retrospective cohort study of 145 patients who underwent cardiac surgery in a tertiary hospital of Athens, Greece, from January to March 2015, was conducted. By using a structured short questionnaire, this study reviewed the electronic hospital database and the medical and nursing patient records for data collection purposes. The statistical significance was two-tailed, and p-values <0.05 were considered significant. The statistical analysis was performed with Mann-Whitney U test and Spearman's correlation coefficient, by using the Statistical Package for Social Sciences software (IBM SPSS 21.0 for Windows). RESULTS: The increased preoperative levels of NLR were associated with significantly higher mortality, both in-hospital (p=0.001) and 30-day (p=0.002), prolonged postoperative hospital length of stay (LOS), both in the cardiac intensive care unit (ICU) (p=0.002), and in-hospital (p=0.018), and likewise with delayed tracheal extubation (p≤0.001). Furthermore, patients with elevated NLR during the second postoperative day had significantly higher in-hospital mortality (p=0.018), increased incidence of pneumonia (p=0.022), higher probability of readmission to the ICU (p=0.002), prolonged ICU LOS (p≤0.001), and delayed tracheal extubation (p≤0.001). CONCLUSION: Increased perioperative NLR seems to be associated with significantly higher mortality and morbidity in cardiac surgery patients. At the same time, NLR is a significant and inexpensive biomarker for the early identification of patients at high risk for complications. In addition, NLR levels could lead clinicians to perform measures for the optimal therapeutic patient approach.