We prospectively studied 169 patients with Crohn's disease to determine if postoperative infectious complications could be related to perioperative blood transfusions. Postoperative septic complications developed in 18 of the 69 patients who received more than 1 unit of blood (26 percent) compared with 8 of the 100 patients (8 percent) who received 1 unit of blood or no blood (p = 0.0014). Previous operation, low body weight, and having an ostomy were also related to septic complications. Patients receiving more than 1 unit of blood were significantly more likely to have low preoperative serum albumin levels, to have undergone abdominoperineal or small bowel resection, and to have an ostomy. Postoperative septic complications were significantly related to perioperative blood transfusions after controlling for these potential confounding factors independently by subgrouping and simultaneously by using multiple logistic regression. Blood transfusion may be a more significant factor in postoperative immune suppression and susceptibility to infection than previously recognized.