BACKGROUND Cardiopulmonary bypass (CPB) contributes to the development of systemic inflammatory response after cardiothoracic surgery. As a measure of inflammation and immune reaction, the neutrophil-to-lymphocyte ratio (NLR) has been linked to poor outcomes in a variety of diseases. However, it remains to be seen whether postoperative NLR is associated with CPB patient mortality. The purpose of this research was to explore the prognostic role of the postoperative NLR in adult patients undergoing cardiothoracic surgery with cardiopulmonary bypass. MATERIAL AND METHODS This is an analysis of data stored in the databases of the MIMIC-III, which contains data of critically ill patients for over 50,000. The exposure of interest was postoperative NLR. The primary outcomeaThis study incorporates data from the MIMIC III database, which includes more than 50 000 critically ill patients. The variable of interest was postoperative NLR. The primary outcome was 30-day mortality and the secondary outcomes were 90-day mortality, length of intensive care unit stay, and length of hospital stay. was 30-day mortality, the secondary outcome was 90-day mortality, length of hospital stay and length of ICU stay. RESULTS We enrolled 575 CPB patients. The ROC curve for the postoperative NLR to estimate mortality was 0.741 (95% confidence interval [CI]: 0.636-0.847, P<0.001), and the critical value was 7.48. There was a significant difference between different postoperative NLR levels in the Kaplan-Meier curve (P=0.045). Furthermore, elevated postoperative NLR was associated with increased hospital mortality (hazard ratio [HR]: 1.1, 95% CI: 1.0-1.1, P=0.021). However, there was no important relationship in these patients between the postoperative NLR levels and 90-day mortality (HR: 1.1, 95% CI: 1.0-1.5, P=0.465). CONCLUSIONS Our findings suggest that higher postoperative NLR is associated with greater hospital mortality in adult patients undergoing cardiopulmonary bypass surgery.