Nine children with histiocytosis X were observed. The course of the disease largely depended upon the degree of dissemination at the time of diagnosis. Bone and skin involvement healed completely with a relatively simple therapy using prednisone in combination either with vincristine or vinblastine. Complete or long lasting partial remissions could be achieved up to involvement of 4 organs. Three patients demonstrating histiocyte infiltrations in 5 organs died. They all were less than 2 years of age. Hyperproliferation of monocytopoiesis was present in all patients manifesting itself by a rise in the fraction of mononuclear phagocytes in bone marrow. This variation was paralleled by monocytosis in the peripheral blood in 5 of 7 cases. Serum muramidase levels were increased in 5 of 6 patients. However, there was no clearcut correlation to the degree of disease dissemination.