This term was coined to describe three closely related disorders characterized by proliferation of the Langerhans cell, a type of histiocyte. Histiocytosis X was the original term used for this group of diseases. The group comprises three well-known disorders and several rarer ones. Letterer-Siwe disease is a rapidly progressive, fatal condition that is seen mostly in early childhood and infancy. There is widespread involvement of bone, skin, visceral, and reticuloendothelial organs by atypical histiocytes, with accompanying anemia and thrombocytopenia. Bone marrow aspiration or, occasionally, lymph node biopsy is the usual diagnostic procedure. Eosinophilic granuloma is the benign member of the triad. It is seen most often in later childhood and most commonly presents as isolated bone lesions. These usually are single but may be multiple. The lungs are occasionally involved. The lesion is composed of histiocytes with many eosinophils and is diagnosed by direct biopsy. Hand-Sch?ller-Christian disease is somewhat intermediate between the other two in terms of chronicity and histology. Bone lesions, often multiple, are the major abnormalities. Soft tissue and reticuloendothelial organs may be affected. There may be very few systemic symptoms, or there may be anemia, leukopenia, and thrombocytopenia. The lesions are composed of histiocytes containing large amounts of cholesterol in the cytoplasm and accompanied by fibrous tissue and varying numbers of eosinophils. Diagnosis usually is by direct biopsy of a lesion.