A 54-year-old woman with epigastric pain had leukocytosis of 73,000/microliter consisting mainly of atypical lymphoid cells with convoluted and cleaved nuclei resembling Sézary cells; the bone marrow aspirate was nondiagnostic. Skin biopsy was unremarkable. The patient also had hypercalcemia and hemolysis with a positive direct Coombs' test, both of short duration. The arterial oxygen tension was decreased, but there was no demonstrable lung pathology. The patient subsequently developed rapidly enlarging lymphadenopathy. Lymph node biopsy was interpreted as "undifferentiated pleomorphic lymphoma." Immunologic functional studies revealed that the majority of the peripheral blood atypical lymphoid cells from involved lymph nodes formed rosettes with sheep erythrocytes. The lymphadenopathy regressed transiently after the administration of chemotherapy and the white blood cell count decreased from a maximum of 385,000/microliter to 3,500/microleter, at which point the arterial oxygen tension returned to normal. The unusual features of this patient are discussed in light of the known characteristics of the various types of T-cell lymphorpoliferative disorders.