Purpose: To assess the interaction of splenectomy and perioperative allogeneic blood transfusions on the prognosis of patients undergoing a potentially curative resection of proximal gastric and gastroesophageal junction (GEJ) cancer, because reports from the transplantation literature demonstrated that the immunosuppressive effects of transfusions are dependent on the presence of an intact spleen. Patients and Methods: Between July 1, 1985, and July 30, 2001, 240 patients underwent complete resection (R0) of a proximal gastric or GEJ cancer (Siewert type II or III). Clinical and pathologic factors were collected in a prospective database. The survival data were modeled using the method of Kaplan and Meier and analyzed by the log-rank test and Cox regression. Results: The median follow-up of the patients was 25 months (40 months for survivors). The median relapse-free survival was 30 months, and the median disease-specific survival was 45 months. Univariate analysis suggested an interaction of splenectomy and perioperative transfusion in their effect on relapse-free survival. Patients who received a perioperative transfusion but did not undergo splenectomy demonstrated the worst prognosis on multivariate analysis independent of other prognostic factors. In patients who underwent splenectomy, perioperative transfusion had no effect on relapse-free survival on multivariate analysis. Conclusion: Our study suggests an interaction of blood transfusion and splenectomy in their effect on survival paralleling the findings in the transplantation literature. The adverse effect of allogeneic blood transfusion on prognosis in patients with gastric cancer seems to be associated with the presence of an intact spleen and is abrogated by its absence.