Histiocytosis X comprises a spectrum of diseases of unknown etiology in which localized or systemic histiocytic proliferations occur, often associated with eosinophilic infiltration of the involved tissues. The three clinical syndromes are: eosinophilic granuloma, Hand-Schuller-Christian disease and Abta-Letteres-Siwesche's disease. Although diagnosis is based on the histochemical findings, the "definite" diagnosis requires the presence of Langerhans' granules in histiocytes. Localized histiocytosis X is treated by surgical excision and irradiation. Corticosteroid treatment is effective in symptomatic control in most patients and a variety of cytotoxic drugs have been used either alone or in combination with corticosteroids.