Acute graft-versus-host disease (GVHD) is thought to derive from direct T-cell injury of target tissues through perforin/granzyme, Fas/FasL interactions, and the effects of inflammatory cytokines. Animal models and some clinical trials support the notion that inhibition of inflammatory mediators such as interleukin-1 (IL-1), tumor necrosis factor α, and interferon γ may ameliorate or prevent GVHD. We hypothesized that blockade of IL-1 during the period of initial T-cell activation would reduce the risk of severe GVHD. We tested this hypothesis in a double-blind, placebo-controlled randomized trial of recombinant human IL-1 receptor antagonist (IL-1Ra) in 186 patients undergoing allogeneic stem cell transplantation. Randomization was stratified by degree of histocompatibility and stem cell source. All patients were conditioned with cyclophosphamide and total body irradiation. GVHD prevention consisted of cyclosporine and methotrexate in all patients. Recombinant human IL-1Ra or saline placebo was given from day −4 to day +10. Randomization was stratified according to GVHD risk. The 2 groups were well-matched for pretreatment characteristics. Moderate to severe GVHD (grades B-D) developed in 57 (61%) of 94 patients receiving IL-1Ra and in 51 (59%) of 86 patients on placebo (P = .88). There was no difference in hematologic recovery, transplantation-related toxicity, event-free survival, or overall survival. We conclude that blockade of IL-1 using IL-1Ra during conditioning and 10 days immediately after transplantation is not sufficient to reduce GVHD or toxicity or to improve survival.