Trial design: all patients with a diagnosis of acute graft-versus-host disease (GvHD) are treated with 6MPred 2 mg/kg per day from day 1 to day 5. Responders have 6MPred tapered to 1.5 mg/kg from day 6 to day 10 and to 1 mg/kg from day 11 to day 15. Nonresponders are randomized to (A) 6MPred 5 mg/kg from day 6 to day 15, or (B) 6MPred 5 mg/kg from day 6 to day 15, and rabbit anti–thymocyte globulin (ATG, thymoglobulin) 1.25 mg/kg on days 6, 8, 10, 12, and 14.