Fatal "postoperative erythroderma" (POE) developed in 2 patients treated with liver lobectomy and the transfusion of fresh blood. Their clinical features and skin-histological findings were indicative of acute graft-versus-host disease (GVHD). The HLA phenotypes of circulating lymphocytes of the 2 patients were heterozygous but became homozygous late in the clinical course and were identical with those of the blood donors. One of the patient's haplotypes was identical with the donor's homozygous haplotype. These findings suggest the mechanism of development of POE in apparently immunocompetent patients. The donor's T lymphocytes are histocompatible with the patient's tissues, are not rejected, and become engrafted. The patient's tissues are not histocompatible with the donor's, so that GVHD develops.