A considerable amount of information has accumulated over the past decade indicating that the transfusion of allogeneic blood products may be associated with adverse effects to the recipient. These include the development of transfusion reactions, TA-GVHD, HLA-alloimmunization, and immunomodulatory effects. The latter might be beneficial for recipients of kidney allografts, in reducing the relapse rate in patients with Crohn's disease, and in ameliorating the rate of recurrence of spontaneous abortion in affected patients; however, the immunosuppressive effects associated with perioperative ABT might adversely affect overall prognosis in patients with a malignancy who undergo curative cancer surgery. In addition, ABT has been shown to be associated with an increased risk for postoperative bacterial infections. The ABT-induced immunomodulatory effects appear to be mediated immunologically by transfused allogeneic passenger leukocytes. The 3 log10 leukocyte reduction of cellular blood products, provided by the currently available commercial leukocyte filters, has been shown to minimize the occurrence of some of the ABT-associated deleterious effects; however, the actual clinical efficacy of leukodepletion has not yet been established, because the available data are largely from retrospective or uncontrolled clinical studies. Properly designed prospective clinical trials are essential to establish optimal conditions for the preparation of blood components destined for clinical use.