Four cases of histiocytosis X (Langerhans' cell granulomatosis) involving the thymus are presented. All the patients were children, their ages at diagnosis ranging from 2 months to 8 years. In two cases, the disease seemed restricted to the thymus; in the other two, there was extrathymic involvement. The treatment was generally in the form of surgical excision. All the patients are alive and well on follow-up. The light-microscopic appearance was that classically described for histiocytosis X in all cases. Staining for S-100 protein was strongly positive in the two cases in which it was carried out.