Blood transfusion is associated with numerous clinical phenomena attributable to immune suppression. Homologous blood transfusion is associated with declines in lymphocyte numbers and inhibition of lymphocyte function. In dialysis patients this immune suppression is accompanied by prolongation of survival of subsequently transplanted allografts. For patients undergoing surgical procedures, the receipt of homologous blood increases the risk of postoperative infectious complications. Patients with malignancies have significantly increased recurrence and mortality rates when removal of their tumor is accompanied by the administration of blood. The clinical course of Crohn's disease may be beneficially influenced by transfusion at the time of resection of diseased bowel. Women suffering recurrent abortion may carry to term following transfusion of spouse leukocytes. Experimental studies, in addition to replicating the clinical studies, have documented that transfusion inhibits wound healing. Blood transfusion, the oldest form of transplantation, causes profound and prolonged alterations in immune function which result in clinical phenomena which can be either beneficial or detrimental to the recipient.