The paradigm related to the immunologic consequences of allogeneic blood transfusions has been extended from humoral allosensitization to the effects of transfusion on cellular immune function. This includes downregulation of effector cells, activation of latent viral infection, and the prolonged circulation of donor immunocompetent cells, as seen in graft-vs-host disease. There are now extensive data showing conclusively that allogeneic transfusions are associated with increases in cancer recurrence rates (80% in colorectal cancer) and postoperative bacterial infections (as much as 200% to 1000% in some studies). Whether these associations are causal or not remains in doubt. Based on animal and clinical studies, we believe these associations are likely, in part, due to immune dysregulation caused by transfusion, perhaps augmented by the effects of hemorrhage, anesthesia, and surgical stress. The most likely mechanism underlying transfusion-induced immunosuppression is anergy due to presentation of large amounts of antigen through the intravenous route. This favors presentation of antigen by "nonprofessional" antigen-presenting cells, a situation that usually leads to anergy or tolerance rather than immune activation. Two additional hypothetical mechanisms proposed for immune dysregulation after allogeneic transfusion are (1) prolonged circulation of donor cells causing subclinical graft-vs-host disease, and (2) reactivation of immunosuppressive viruses latently present in recipient white blood cells. Results of some initial interventional studies, employing autologous transfusions or removing white blood cells from allogenic donor blood suggest that relatively simple, cost-effective strategies to ameliorate these complications may be at hand.