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Introduction Colorectal cancer (CRC) is the third most common cancer and the second most common cause of cancer death. Anaemia is common in perioperative CRC and studies have shown postoperative anaemia is associated with increased mortality. In addition, large cohort studies and meta-analyses show chemotherapy beyond 6-8 weeks following resection is associated with reduced disease-free survival. The aim of this study to is to investigate the effect of postoperative anaemia on patient outcomes including timely receipt of adjuvant chemotherapy, cancer recurrence and mortality.Methods A retrospective cohort study was conducted using linked primary care data (CPRD) and hospital episode statistics (HES) from 2000-2022. Patients with potentially curative CRC and haemoglobin (Hb) results from primary care who did not receive neoadjuvant chemotherapy were identified using OPCS4 and SNOMED-CT codes, with a final cohort of 12,773 patients.Cox regression models were constructed to analyse cancer recurrence and disease-free survival. Logistic regression was used to identify risk factors specific to delayed chemotherapy. Models corrected for preoperative anaemia, which has been associated with stage of disease, age, comorbidity, gender, year of surgery and deprivation level. Postoperative Hb was described by quintile to identify the impact of Hb ranges on patient outcomes.Results We demonstrate that postoperative anaemia is associated with reduced disease-free survival (higher HR of death and/or recurrence). Hb <102g/dl, (HR 1.644 (1.479-1.828)) p<0.001, Hb 101-110g/dl, (HR 1.399 (1.256-1.559)) p<0.001, and Hb 111-118g/dl (HR 1.221 (1.092-1.365)) p<0.001. We also illustrate that severe postoperative anaemia increases the likelihood of cancer recurrence, Hb <102g/dl HR 1.202 (1.079-1.339) p=0.001.We report that the likelihood of patients receiving adjuvant chemotherapy within 8 weeks of surgery reduced in a stepwise fashion with worsening postoperative anaemia. Hb<101g/l, OR 1.791, p< 0.001, Hb 102-110g/l, OR1.363, p=0.020 and Hb 111-118g/l, OR1.454, p=0.004. We show similar results for patients waiting >12 weeks and found that deprivation was associated with severely delayed chemotherapy, HR1.44 (1.05-1.99) p=0.024.Conclusions We are the first group to demonstrate that postoperative anaemia is associated with delayed receipt of adjuvant chemotherapy, increased cancer recurrence and reduced disease-free survival. Whilst other groups have shown increased all-cause mortality, which we also demonstrate, we have elucidated the likely cause for increased mortality. Considering these findings, we believe that postoperative anaemia should be more aggressively treated in CRC patients. We propose further research including a definite clinical trial with adequate follow-up to investigate receipt of chemotherapy.