The clinical pathologic syndrome of graft-versus-host disease (GVHD) is usually a sequela of bone marrow transplantation. This disorder occurs as a result of recognition by engrafted donor-derived lymphocytes of "foreign" recipient transplantation antigens. GVHD may also result from engraftment of lymphocytes from other sources, including (1) transfusion of lymphocytes containing blood components, (2) transplacental maternal fetal transfusion, and (3) passive transfer of lymphocytes in solid organ transplantation. The recipients are usually severely immunodeficient and thus incapable of rejecting the transfused lymphocytes. This syndrome may, however, also develop in immunologically competent patients receiving blood products from individuals with histocompatibility antigens not recognized as foreign.