Recent laboratory investigations of patients with histiocytosis-X and their pathologic tissues demonstrate the close immunologic relationship between the Langerhans-like cells of these patients and normal epidermal Langerhans cells of the mononuclear phagocytic system. A deficiency of a peripheral blood lymphocyte subpopulation is seen in histiocytosis-X patients. Its relevance to the pathogenesis of the disease is yet to be determined. The granuloma of histiocytosis-X contains large numbers of cells similar to epidermal Langerhans cells, components of the mononuclear phagocytic system. The Langerhans cells of histiocytosis-X can now be characterized by a number of immunologic and immunohistochemical techniques. Similarities and differences between them and other components of the mononuclear phagocytic system have been recognized, supporting the concept of the duality of the system's proliferative disorders. There is a deficiency of suppressor T lymphocytes in patients with active histiocytosis-X and a rise in the number of suppressor cells after in vitro incubation with thymic hormones. Increases in the peripheral blood suppressor cell number occur spontaneously or inconsistently after parenteral administration of thymic hormone. It is not known at present what the relationship is between the decrease in suppressor lymphocytes and the reports of thymic abnormalities in these patients. More investigations are needed in patients with active and inactive disease: measuring serum thymic factors, performing clinical trials with thymic products, determining monocyte or macrophage function, and evaluating other possible dysfunctions of suppressor T lymphocytes.