A group of 548 patients was analyzed retrospectively to determine risk factors, particularly the perioperative administration of blood, for postoperative septic complications after intra-abdominal operations. Of the entire group, 198 (36.1 per cent) had at least one complication. The postoperative mortality rate was 0.9 per cent. Using univariate analysis, blood transfusion, serum protein concentration, gastric malignant disease, total gastrectomy, anastomotic dehiscence, diabetes mellitus, the surgeon and perioperative antibiotics were significant factors influencing the rate of postoperative septic complications. After adjustment for all these factors, logistic regression analysis was used to study the effect of perioperative blood transfusion. Age, serum protein concentration, antibiotics, extended malignant disease of the colon, gastric operation, total colectomy, operation performed by staff members and anastomotic dehiscence were significant independent factors. Blood transfusion per se was not significant (p = 0.07). In patients who received more than 3 units of blood, the postoperative septic complication rate was significantly elevated (p = 0.003). We conclude that the perioperative administration of blood may negatively influence the risk of developing postoperative septic complications after intra-abdominal operations.