From a surgical point of view, one of the most important consequences of blood transfusion is the immunosuppressive effect and its possible link to postoperative infectious complications and cancer recurrence. The underlying mechanism of transfusion-induced immunosuppression seems to be mediated by transfusion with leukocytes. In prospective randomized trials, therefore, efforts have been made to control the immunosuppressive effect of allogeneic blood transfusion by the use of leukocyte-reduced blood. Only one prospective clinical trial has asserted the effect of leukocyte reduction on cancer recurrence, and no benefit to outcome was found. In four big randomized, prospective clinical trials, allogeneic blood transfusion was asssociated with significantly increased rates of postoperative infections, and the use of leukocyte reduction was found to have striking benefits in three of the trials, with a several-fold reduction in rates of morbidity and mortality.