In addition to organ transplantation, the immunomodulating effect of allogeneic blood transfusion might also have consequences for patients with malignancies or other surgical patients requiring blood transfusion. It has been suggested that this effect of allogeneic blood transfusion could adversely affect the prognosis of patients with a malignant disorder or increase the susceptibility for postoperative infections. These deleterious effects of allogeneic blood transfusion, which are suspected to be mediated by the induction of immunosuppression are further discussed in this review. We conclude that, whenever a detrimental effect of blood transfusions on cancer prognosis or the incidence of postoperative infectious complications exists, this relationship is not causal, but reflects the circumstances which necessitate the transfusion.