A 63-year-old white man had a history of recurrent pneumonia, pancytopenia, and splenomegaly when the diagnosis of hairy cell leukemia was made on bone marrow biopsy examination. Splenectomy confirmed that diagnosis and his pancytopenia moderately improved. Three years following the diagnosis, the patient developed an upper abdominal mass involving the stomach wall that was found to be an anaplastic "large cell" neoplasm. Palliative radiotherapy was started, but the patient died 2 months later. Cytochemical studies of the anaplastic gastric neoplasm revealed cytoplasmic tartrate resistant acid phosphatase activity. Electron microscopy showed no epithelial differentiation. These observations suggest that the gastric neoplasm represented an evolution of hairy cell leukemia into a more aggressive tumor analogous to the transformation that occurs in other B-cell neoplasms.

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