Clinicopathological features of three male cases with malignant histiocytosis (MH) are described. Case 1, aged 27, had had an indurated swelling of the left mandibular region, histologically being chronic lymphocytic inflammation, with ebb and flow for 4 years prior to the onset of MH, with low grade fever, lymphadenopathy and pulmonary infiltration. Histology of the lymph node was compatible with MH, and only a temporary improvement was obtained by COP therapy. Case 2, aged 32, showed acute febrile onset with severe anemia and splenomegaly. Diagnosis of MH was determined by bone marrow histology. COP therapy appeared effective, but caused severe leukopenia and thrombocytopenia resulting in fatal gastrointestinal bleeding. Case 3, aged 16, had high fever and cutaneous mass of the left chest wall, histology of which suggested MH. Bone marrow biopsy was also diagnostic. Severe pancytopenia allowed only a limited therapy. Morphology of the histiocytes was variable in each case. Diffuse infiltration of neoplastic histiocytes in many organs and erythrophagocytosis in the bone marrow were commonly found in all the cases. The present cases suggested a diagnostic value of bone marrow biopsy and possible effect of antineoplastic combination therapy on earlier stage of MH.