Neoplasias of the monocyte/macrophage system are rare. They include malignant histiocytosis, sarcomas of dendritic cells, and possibly a malignant form of Langerhans cell histiocytosis. Characterization of all these neoplasms can be easily done along the lines which have been developed for the derivation of the normal cellular counterparts. The possibility of a malignant transformation of the cells of the monocyte/macrophages system is suggested by the fact that all members of this system can undergo mitotic division. The most controversial among the different entities is malignant histiocytosis, which has proved to be anaplastic large cell lymphoma (ALCL) with Ki1 expression in most cases. The few cases of true malignant histiocytosis may or may not express the Ki1 antigen. Sarcomas of dendritic cells have been related mainly to interdigitating or follicular dendritic cells. Prognosis of follicular dendritic cell sarcoma is probably more favorable than that of interdigitating cell sarcoma. Besides these more frequent subtypes, a sarcoma of the sinus lining cells can be separated which represents a low-grade malignant neoplasm. All cases encountered so far were associated with a protracted clinical course, albeit multiple recurrences. A distinctly more aggressive behavior is encountered in those cases which have been identified as a malignant form of Langerhans cell histiocytosis. However, the issue of this particular form of histiocytic disorder is still unsettled.