We present an immunocompetent patient with transfusion-associated graft-versus-host disease (GVHD), in which chimaerism of peripheral blood lymphocytes was demonstrated by analysis of a highly polymorphic genome. The patient was treated successfully with anti-CD3 monoclonal antibody, OKT3 and cyclosporin A. Although it is undoubtedly important to prevent transfusion-associated GVHD by irradiation of cellular blood components, intensive therapy with OKT3 and cyclosporin A in the early phase of onset may be effective for treatment of this potentially fatal condition. The mechanism of the effectiveness of this treatment for transfusion-associated GVHD is discussed.