Langerhans cell histiocytosis in the adult is rare, but it is important to recognize its occurrence, as it must be differentiated from lymphoma, myeloma, and a variety of skin conditions and endocrinopathies. It has been reported in patients up to the ninth decade of life, and occurs equally in men and women. Local disease has a good prognosis, but associated diseases--particularly malignancy--may be the cause of death in some adults. The optimal treatment is not known. Coordinated investigation of the epidemiology and therapy of this disease is needed.