Acute graft versus host disease (GVHD) remains one of the most significant and potentially lethal complications of allogeneic bone marrow transplantation. Depending upon the type of transplant, the incidence of acute GVHD varies between 20 - 50% in related donor transplants, or as high as 70 - 90% in unrelated donor transplants. Acute GVHD affects the skin, liver and gastrointestinal (GI) tract and usually occurs within 20 - 40 days of the bone marrow infusion.