The immunomodulative effect of perioperative allogeneic blood transfusion on host immunocompetence was studied in 109 patients with gastric cancer at various stages. Mitogen-induced lymphocyte blastogenesis, lymphocyte surface markers (specific for T, B, CD4, and CD8 populations), and the activity of natural killer (NK) cells were examined before surgery and then 2 and 4 weeks after surgery. The effects on host immunocompetence of transfusion alone, in the absence of any effect of surgical stress, were studied, preoperatively, in nine patients who received preoperative transfusion. Although a tendency towards a decrease in the posttransfusion activity of NK cells was observed, there were no statistically significant differences between pre- and posttransfusion levels. Mitogen-induced blastogenesis and the activity of NK cells were significantly impaired 2 weeks after surgery in transfused patients as compared to those in nontransfused patients with gastric cancer at stage III and stage IV, and postoperative survival was significantly lower in transfused as compared to nontransfused patients. These results indicate that perioperative allogeneic blood transfusion exacerbates surgical stress-induced postoperative immunosuppression and has a negative effect on prognosis in patients with gastric cancer.