Homologous blood transfusions have been associated in both animals and humans with an increased risk of acute postoperative infectious complications. Eighty-four patients who underwent hip replacement surgery and were transfused with 2 or 3 units of blood were analyzed to determine whether those receiving homologous transfusions had different outcomes than those receiving autologous blood only. Only patients free of other risks for postoperative infection were studied. Those receiving homologous blood had a 32 percent (16/50) rate of proven or suspected infections, which was significantly higher than the 3 percent (1/34) rate in patients receiving autologous blood (p = 0.0029). Wound infections accounted for only a minority (6/17) of the proven or suspected infections, which suggests that nonsurgical factors contributed to these complications. The patients identified as being infected required significantly more antibiotic therapy (mean, 7.6 days) and lengthier hospital stays (mean, 15.5 days) than the patients who remained free of evidence of infection (means: 2.3 days of antibiotics and 12.3 days in the hospital) (p = 0.0001 for each variable). Other potential risk factors for infection, such as duration of surgical procedure, advanced patient age, amount of blood loss, type of anesthesia, surgeon performing the operation, use of a cemented versus porous-coat prosthesis, leukocytopenia, anemia, and underlying medical diagnosis, did not account for the differences in infection rates seen in those receiving homologous and autologous transfusions. These results confirm previous reports of an increased risk of postoperative infection in patients receiving homologous transfusions. Homologous transfusion may contribute to an increased risk of infection by immunologic modulation of the recipient.(ABSTRACT TRUNCATED AT 250 WORDS)