Abstract: The ultrastructual changes of tumor cells of specific lesions of 3 patients with cutaneous malignant histiocytosis are presented in this study. The tumor cells were divided into three types: well differentiated histiocytes, pooly differentiated histiocytes and atypical histiocytes. The latter could be subdivided into giant nuclear and irregular nuclear histiocytes. The phagocytic function of atypical lustiocytes was lower than that of well differentiated histiocytes. In tumor cells the debris of cytoplasm which could not be found by light micresopy, was observed easily by electron microscopy.