AbstractBackgroundAnemia has negative effects on long‐term outcomes of rectal cancer patients; however, its status as a risk factor for severe complications is disputed. Perioperative risks may differ based on the severity of pre‐surgical anemia; nonetheless, no previous study has investigated these differences. This study identified risks of severe postoperative complications in rectal cancer patients based on severity of their pre‐surgical anemia.Materials and MethodsThis study enrolled patients who underwent low anterior resection for rectal cancer and were registered in the Japanese National Clinical Database (NCD) between 2017 and 2019. Anemia severity was categorized into three levels: mild, moderate, and severe. A logistic regression model was applied to calculate the risk‐adjusted odds ratio (OR) of severe complications after surgery.ResultsThis study analyzed a cohort of 51 765 rectal cancer patients who underwent low anterior resection. Results showed that severe complications occurred in 10.9% of patients and were significantly more frequent in patients with anemia (13.6%) than those with normal hemoglobin levels (9.2%). Risk‐adjusted ORs of severe complications in the severe, moderate, and mild anemia groups versus the normal group for males were 1.19 (95% confidence interval [CI]: 0.89–1.58), 1.47 (1.34–1.62), and 1.21 (1.12–1.31), respectively. Those for females were 1.39 (0.90–2.15), 1.64 (1.37–1.97), and 1.36 (1.16–1.58), respectively.ConclusionsAccording to this large cohort study, pre‐surgical anemia significantly increases the risk of severe postoperative complications in rectal cancer patients. Even mild anemia presents a significant risk.