Graft-versus-host disease (GVHD) results in multiorgan system (skin, intestine, upper gastrointestinal tract, and liver) damage caused by donorderived alloreactive T lymphocytes and is a sigtllficant source of morbidity and mortality after bone marrow transplantation (BMT).'" Despite advances in preventing GVHD in humans by combination in PiPo chemotherapeutic prophylactic agents (eg, methotrexate plus cyclosporin or methotrexate, anti-thymocyte globulin, cyclosporin), GVHD remains a sigtllficant cause of morbidity and mortality following BMT. 5-7 We have reviewed the incidence and severity of GVHD at our institution and have noted that 46 percent of 469 recipients of histocompatible sibling donor grafts transplanted between 1979-1987 developed clinical grade 11-IV (moderate/severe) acute GVHDa7 In adults (2 18 years of age), 63 percent developed grades 11-IV GVHD. Both the incidence and severity of acute GVHD are sigtllficantly increased if unrelated individuals rather than histocompatible siblings are used as bone marrow don~ rs.~,~ In a recent review of GVHD in 52 recipients of unrelated donor transplants, grade 11-IV GVHD was observed in 79 percent, a sigtllficantly higher propomon than noted in 102 recipients of histocompatible sibling donor grafts, who had a 36 percent incidence of GVHD. 8