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Less than 200 cases of malignant histiocytosis (histiocytic medullary reticulosis) have been reported in the literature. Five previously reported cases and the case reported in this article have been preceded by acute lymphocytic leukemia. All of these cases have had similar courses characterized by prompt response of the leukemia to chemotherapy followed in three to six months by the onset of rapidly fatal malignant histiocytosis. The leukemic lymphoblasts in the two cases studied for lymphocyte surface markers had T cell markers. No residual leukemia was identified at autopsy in four of the six cases.