Fatal graft-versus-host disease (GVHD) developed in a patient with Hodgkin's disease treated with combined chemotherapy and radiotherapy following the transfusion of 2 U of packed red blood cells. Clinical features of the GVHD included the development of exfoliative dermatitis, progressive hepatic dysfunction, aplastic anemia, and finally progressive fatal pneumonia. GVHD was documented by skin biopsy and chimerism by HLA typing. The HLA phenotype of the patient's skin fibroblasts [A3, Bw44 (w4)/A2, B15 (w4)] was appropriate for parental haplotypes and probably represented her true HLA phenotype. Lymphocytes from the patient (peripheral blood and lymph node biopsy) were of a different HLA phenotype (A3; Bw35, w38, w4, w6; Cw4), which was inappropriate for parental HLA haplotypes but identical to the HLA phenotype of one of the blood donors. The HLA-DR typing of the patient's family and of the blood donor demonstrated that the patient and the donor probably were HLA-DR identical (DRw5/DRw6), although no B lymphocytes could be obtained from the patient for direct DR typing. We are currently irradiating all blood products administered to patients with Hodgkin's disease receiving intensive treatment. Further observations will be necessary to determine whether transfusions to other cancer patients with immunodeficiency states should be restricted to irradiated blood products.