A 66-year-old man was admitted for shortness of breath and showed fever, abdominal fullness and paraplegia. Monocytosis amounting to 25% and an elevation of serum LDH (4,281 mIu), were remarkable in the laboratory findings. He died of pulmonary insufficiency about a month after admission. On autopsy hepatomegaly (1950 g), splenomegaly (780 g), but no lymphadenopathy and small infarction in the thoracic spinal cord causing paraplegia was noted. Histopathologically, the invasion of the tumor cells into the liver, spleen, lymph nodes, bone marrow and other organs was observed. Malignant histiocytosis was diagnosed by histologic and immunohistochemical studies (lysozyme positive, S-100 protein negative).