Malignant histiocytosis as a terminal condition in chronic lymphocytic leukemia.

Chronic lymphocytic leukemia rarely develops an acute blastic phase as a terminal complication. When it does, the blastic phase is characterized by the appearance of large, immature cells with the same immunologic markers as the cells in the chronic phase of chronic lymphocytic leukemia; this coexistence of mature lymphocytes and blastic forms in chronic lymphocytic leukemia has been termed "Richter's syndrome." In a patient of ours with chronic lymphocytic leukemia, the development of a presumed large cell lymphoma was recognized on both bone marrow aspiration and biopsy examination, but later the condition was proved to be a true histiocytic neoplasm (malignant histiocytosis) by cytochemical and immunohistochemical techniques. We are unaware of another reported instance of malignant histiocytosis as a terminal complication of chronic lymphocytic leukemia and mimicking Richter's syndrome.