The bulk of experimental and clinical data support the theory that homologous transfusion causes significant down-regulation of immunologic functions in a number of settings. These changes in immune function may account for the beneficial associations of transfusion with increased renal allograft survival, and decreased recurrence in Crohn's disease. Conversely, these transfusion-induced effects may be responsible in part for the deleterious association of homologous transfusion with increased cancer recurrence, and increased posttransfusion bacterial and viral infection rates. Host defenses against malignancy and infection may in some instances be severely compromised by transfusions of homologous blood, but the circumstances under which this occurs need to be better defined. Likewise, the hypothesis that modification of blood components to contain fewer leukocytes or less plasma might ameliorate these effects is attractive, but little or no data exist to support or refute it. Future clinical studies will no doubt address these issues.