Acute graft-versus-host disease (aGVHD), one of the most important complications of allogeneic hematopoietic stem cell transplantation (HSCT) is associated with significant morbidity and mortality. Grades II to IV aGVHD occur in 30% to 50% of matched related donor recipients and 50% to 70% of unrelated donor recipients. Standard first-line treatment consists of methylprednisolone at a dose of 2 mg/kg/d or equivalent that produces response rates of 63% to 95% for grade II, 17% to 39% for grade III, and 0% to 6% for grade IV aGVHD. In aggregate, approximately 40% to 50% of patients with acute GVHD experience a complete or partial response with primary therapy, whereas 20% to 60% require salvage treatments. The 1994 consensus conference on acute GVHD grading reported 100-day survival rates of 78% to 90% with grade I, 66% to 92% with grade II, 29% to 62% for grade III, and 23% to 25% for grade IV. The higher mortality rates were directly attributable to acute GVHD or to the subsequent immunosuppression from high-dose corticosteroids and other medications required for treatment of GVHD.