Background: The perioperative risk factors that cause severe morbidity and prolongation of postoperative hospital stay after cardiac surgery should be determined. Various scores have been used to predict morbidity and mortality. Preoperative blood counts are considered potential biomarkers of inflammation and oxidative stress. Inflammatory and immune imbalances may have a significant impact on postoperative adverse events. The present study aimed to investigate the association and potential predictive properties of red cell distribution width/ lymphocyte ratio (RLR) for major adverse events in adult patients who underwent coronary surgery with cardiopulmonary bypass. Methods: After approval from the ethics committee, pre- and post-operative data of 700 patients were obtained from the electronic database of the hospital, intra- and post-operative anesthesia, and intensive care unit follow-up charts. We performed a stepwise multiple logistic regression analysis to investigate the association of RLR with major adverse events in adult patients who underwent coronary surgery with cardiopulmonary bypass. Results: Among 700 patients, 47 (6.7%) had major adverse events after surgery. Multivariate logistic regression analysis showed that age (odds ratio [OR], 1.08; 95% confidence interval [CI], 1.03–1.12; P < 0.001), mean platelet volume (OR, 1.49; 95% CI, 1.07–2.06; P = 0.017), and RLR (OR, 1.21; 95% CI, 1.02–1.43; P = 0.026) were significantly associated with major adverse events. Conclusions RLR indicates the balance between inflammatory and immune responses. Therefore, it can be used to predict adverse events following coronary surgery.