We reviewed clinical and pathologic characteristics of 18 cases of malignant histiocytosis including 2 autopsy cases with special interest on their immunohistochemical characteristics. We report 3 cases of unusual immunohistochemical finding and postulate these cases may be a supportive evidence of the view that dendritic cells share common precursor with those of mononuclear phagocytic system. More accumulated cases in the future and careful analysis would be needed to answer the basic question about the origin of malignant histiocytosis.