The clinical, morphologic, and cytochemical findings of 7 patients with malignant histiocytosis are presented in this study. The diagnoses were confirmed by the use of electron microscopy and cytochemistry (acid phosphatase and naphthol-AS-acetate-esterase). Two different types of malignant histiocytes were identified by the differentiation and development of cell organells. The relation of malignant histiocytosis to monocytic leukemia and to normal histiocytes (and their precursors) is discussed. And finally, there are statements on the origin and peculiarities of malignant histiocytes.