The histiocytoses represent a heterogeneous group of conditions. Their common denominator is the proliferation and the activation of the mononuclear phagocyte system (MPS). On the basis of recent advances in the knowledge of the distribution, biology, and behavior of the MPS, the following classification is proposed. Reactive and secondary histiocytoses related either to a chronic parasitic intracellular infection or to a patent or latent immunodeficiency state. Some well-established conditions belong to this category--i.e., familial lympho-histiocytosis, cytophagic sinus histiocytosis, Omenn's reticulosis. The dystrophic histiocytoses associated with the storage of either exogenous or endogenous material. It is prudent to separate the storages of homogeneous and chemically defined lipid material (such as cerebroside, sphingomyelin, etc.) from those of heterogeneous lipid material. Proliferative histiocytoses: it is crucial to distinguish the malignant histiocytosis from the histiocytosis X, which seems to be associated with a nonmalignant proliferation of a subpopulation of the MPS, the Langerhans cell system.