Seventy-seven patients (age 12 to 46 years) who underwent allogeneic marrow transplantation for hematologic malignancy or aplastic anemia and who had grade II to IV acute graft-versus-host disease despite methotrexate prophylaxis were randomly assigned to receive methylprednisolone 2 mg/kg per day intravenously (n = 39) or cyclosporine (n = 38) either 12 to 15 mg/kg per day orally or 3 to 5 mg/kg per day intravenously. In both groups, clinical and histologic evidence of graft-versus-host disease was detected at medians of 16 and 25 days, respectively. Drugs were given for a minimum of 14 days unless significant deterioration occurred. If graft-versus-host disease did not improve with this therapy, treatment with a second agent was initiated. Treatment responses were scored after reviewing clinical and laboratory data collected before, during, and after the 14-day treatment period. Possible scores were as follows: -1, worse; 0, no change; + 1, improvement in one organ system (skin, liver, gut) with no deterioration in the other two; +2, complete resolution of all involved systems. The median response score among 39 methylprednisolone-treated patients was 0. Sixteen patients (41 percent) showed response to treatment, 11 with partial and five with complete response. The median response score among 38 cyclosporine-treated patients was +1. Twenty-three patients (61 percent) showed response to treatment, 15 with partial and eight with complete response (p = 0.039). Twenty patients receiving methylprednisolone and 18 receiving cyclosporine required additional therapy. The incidence of chronic graft-versus-host disease was similar in both groups. It developed in all nonresponding patients at risk who had received secondary therapy. Among responding patients (scores +1 or +2) who were not given additional treatment, chronic graft-versus-host disease developed in eight of 11 (72 percent) receiving methylprednisolone and five of ten (50 percent) receiving cyclosporine. Survival beyond 17 months was similar in the two groups (28 percent and 24 percent, respectively). These data suggest that cyclosporine is a useful agent for the treatment of acute graft-versus-host disease, comparable in its efficacy to methylprednisolone.