Two patients with acute leukemia were treated with chemoradiotherapy and allogeneic bone marrow transplantation. Despite the prophylactic use of methotrexate after grafting, both patients developed severe graft-versus-host disease that was refractory to treatment with methylprednisolone. The graft-versus-host disease was then treated with a monoclonal antibody, 64.1, that reacts with a p19 antigen on human T cells. The disease responded dramatically to this treatment, but both patients subsequently developed a fatal polyclonal lymphoproliferative disorder arising in donor-derived B cells. Hybridization studies showed Epstein-Barr virus in both tumors. The combined effect of severe end-stage graft-versus-host disease and potent immunosuppressive therapy probably resulted in a progressive immunodeficiency syndrome that abrogated the T-cell-mediated surveillance mechanism that normally modulates the proliferation of Epstein-Barr-virus-infected B lymphocytes.