Thirty-one autopsy cases previously diagnosed as malignant histiocytosis (MH) were studied by means of immunohistochemical staining. Antibodies detecting the formalin resistant epitopes on T-cells, B-cells and those of histiocyte/monocyte origin were used. It was shown that the malignant histiocytes reacted only to the cell markers derived from histiocyte/monocyte, and only a part of lymphocytes showed positive reaction to the T and B cell markers. It is suggested that the histiocyte/monocyte lineage is the possible origin of the malignant proliferating cells in MH. The clinicopathological features and the differentiation of MH from familial erythrophagocytic lymphohistiocytosis, virus-associated hemophagocytic syndrome and malignant lymphoma are described. The pathogenesis, the causes of death and the points for attention in the treatment of MH are also discussed.