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Homologous blood transfusion has been associated with an increased risk of postoperative infectious complications.To test the clinical consequences of this apparently immunosuppressive effect of homologous blood in a controlled trial, we designed a study in which the control group deposited autologous blood before their operations for use should transfusion be needed.We enrolled 120 patients with apparently curable colorectal cancer who were able to predeposit autologous blood (haemoglobin >12.5 g/dL). 58 patients were assigned to receive homologous blood if blood transfusions were needed during operation, and the other 62 to receive their own predeposited blood followed, if necessary, by homologous blood. Despite the similarity between the groups in factors known to affect the risk of postoperative infections, there was a significant difference in postoperative infection rate between the homologous and autologous blood groups (17 (27%) vs 7 (12%), p<0.05; unadjusted odds ratio 2.75 (95% CI 1.07-7.11). The rates of non-infectious complications were similar. Probably because their preoperative blood depositing caused the autologous blood patients to have lower haemoglobin concentrations, they were more likely to require transfusion than were the homologous blood group (53 (91%) vs 37 (60%), p<0.001; relative risk 1.53 (1.24-1.89)). 20 (35%) required homologous as well as autologous blood. To adjust for the many infection-related factors, we did multivariate regression analysis; tumour location, preoperative ASA index, and study group assignment were the only significant risk factors. The odds ratio for postoperative infections adjusted for these factors was 2.84 (1.02-7.98, homologous vs autologous). Testing of delayed-type hypersensitivity responses before and after surgery showed decreases in both mean diameter and number of positive reactions in recipients of homologous blood and slight increases in those who received autologous blood.This study shows the clinical potential of blood-transfusion-mediated immunomodulation, which may be important also in tumour immunology.