Background. The present study investigated relationships between perioperative blood transfusion, postoperative systemic inflammatory response, and outcomes following surgery for colorectal cancer. Methods. Data were recorded for patients (n = 544) undergoing potentially curative, elective surgery for colorectal cancer at a single center between 2012 and 2017. Transfusion history was obtained retrospectively from electronic records. Associations between blood transfusion, postoperative C-reactive protein (CRP), albumin, hemoglobin, complications, cancer-specific survival and overall survival (OS) were assessed using propensity score matching (n =116). Results. Of 544 patients, the majority were male (n =294, 54%), over 65 years of age (n =350, 64%), and with colonic (n =347, 64%) node-negative disease (n =353, 65%). Eighty-six patients (16%) required perioperative blood transfusion. In the unmatched cohort, blood transfusion was associated with higher median postoperative day (POD) 3 CRP {143 [interquartile range (IQR) 96-221 mg/L] vs. 120 (IQR 72-188 mg/L); p = 0.004}, lower median POD 3 albumin [24 (IQR 20-26 g/L) vs. 27 (IQR 24-30 g/L); p \ 0.001], more postoperative complications [odds ratio (OR) 3.28, 95% confidence interval (CI) 2.03-5.29] and poorer OS [hazard ratio (HR) 3.18, 95% CI 2.08-4.84]. In the propensity score matched cohort, blood transfusion was similarly associated with higher median POD 3 CRP [130 (IQR 93-196 mg/L) vs. 113 (IQR 66-173 mg/L); p = 0.046], lower median POD 3 albumin [24 (IQR 20-26 g/L) vs. 26 (IQR 24-30 g/L); p \ 0.001], more postoperative complications (OR 2.91, 95% CI 1.36-6.20) and poorer OS (HR 2.38, 95% CI 0.99-5.73). Conclusions. Perioperative blood transfusion was associated with postoperative inflammation, complications, and poorer survival in patients undergoing colorectal cancer surgery, with and without propensity score techniques. A significant proportion of patients undergoing surgery for colorectal cancer require allogeneic blood transfusion in the perioperative period. 1 Such transfusions are associated with infective postoperative complications and anastomotic leak. 2,3 They are also associated with disease recurrence, 3,4 and this effect is even greater in the presence of infective complications. 5 Therefore, it has long been hypothesized that allogeneic blood transfusion might impair the host adaptive immune response to both pathogens and circulating or micrometastatic tumor cells. 6 There is increasing evidence that an exaggerated postoperative systemic inflammatory response following surgery for colorectal cancer is associated with postoperative complications and long-term survival. 7 Furthermore, there is some observational evidence that modulation of this response can improve both short-8 , and long-term outcomes. 9 It may be that perioperative blood transfusion and postoperative complications have a negative impact on oncologic outcomes via a common pathway, the systemic inflammatory response. 10