An erythrophagocytic neoplastic infiltration of the bone marrow which resembled malignant histiocytosis was found in an elderly man who, when initially examined, had fever and cytopenias. Results of cytochemical and immunologic studies were consistent with an acute lymphocytic leukemia in which the lymphoblasts showed the simultaneous expression of lymphoid stem cell and B-cell markers. Chromosome analysis revealed an abnormal clone with a deletion of part of the long arm of chromosome 20. This case illustrates (1) the occurrence of striking erythrophagocytosis by lymphoblasts at the time of initial presentation of a patient with acute lymphocytic leukemia, (2) the fact that abnormalities of chromosome 20 can occur in patients with acute lymphocytic leukemia, and (3) the capability of lymphoid malignant lesions to show the simultaneous expression of antigens that are characteristic of different stages of lymphoid differentiation.
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