This prospective randomized trial compared the effect of antihuman thymocyte globulin (ATG) versus corticosteroids as treatment of graft-versus-host disease (GVHD) in recipients of HLA-identical allogeneic bone marrow transplants. Patients undergoing transplantation as therapy for either hematologic malignancies or aplastic anemia were given methotrexate as postgrafting immunosuppression. Patients who nevertheless developed acute GVHD of moderate severity were randomized to receive either corticosteroid therapy or ATG therapy. Thirty-seven patients were randomized: 20 patients received corticosteroids, and 17 received ATG. Both ATG and corticosteroids were in general well tolerated, although all patients receiving ATG developed fever and chills. Both treatment modalities were associated with a mild decrease in severity of GVHD after therapy. There was, however, no significant difference between treatment groups, whether assessed by improvement in specific organ involvement, improvement in the overall grade of GVHD, need for additional therapy for acute GVHD, or the proportion of patients who developed chronic GVHD. Infectious complications and survival were also not different between treatment groups. Thus, corticosteroids were as effective as ATG for the treatment of acute GVHD in recipients of HLA-identical marrow transplants and, therefore, appear to be a reasonable choice as primary therapy for acute GVHD.