Objective To study the effects of allogeneic blood transfusion on expression of CD40L on peripheral blood monocytes (PBMCs) in patients undergoing radical gastrectomy for gastric carcinoma. Methods Thirty patients classified as ASA physical status Ⅰ- Ⅱ undergoing radical gastrectomy for gastric carcinoma were randomly divided into three groups with 10 patients in each group : group A received no allogeneic blood; group B received leukodepleted blood; group C received allogeneic whole blood during operation or within 12 h after operation. The patients were premedicated with intramuscular phenobarbital sodium 0.1 g and atropine 0.5 mg. Anesthesia was induced with midazolam 0.06 mg ? kg-1 , fentanyl 2 ?g ?kg-1 , propofol 1.5 mg ?kg-1 and vecuronium 8mg and maintained with continuous infusion of propofol (100?g?kg-1?min-1) and vecuronium (5?g ?kg-1?min-1) and intermittent inhalation of 1l%-2% isoflurane. The patients were mechanically ventilated after tracheal intubation. Blood samples were taken before operation and 2, 5 and 10 days after operation. The PBMCs and plasma were separated from peripheral blood by Ficoll-Hypaque centrifugation. The PBMCs were washed and incubated with the patients own plasma (final-concentration 10% ) and PHA (final concentration 20 ?g?ml-1 ) at 37℃ in a 5% CO2 atmosphere for 48h. CD40Lexpression on PBMCs was quantified by flow-cytometry.Results The demographic data including sex, age, bodyweight and duration of operation and intraoperative blood loss were comparable among the three groups. There was no significant difference in the CD40L expression before operation among the 3 groups. In group A there was no change in CD40L expression after operation. In group B CD40L expression on PBMCs increased significantly on the 2nd postoperative day, but returned to preoperative level on the 5th postoperative day. In group C the CD40L expression on PBMCs kept increasing on the 2nd and 5th postoperative day and did not return to preoperative level on the 10th day. The increase in CD40L expression was significantly larger in group C than that in group B ( P