Histiocytosis X.

Histiocytosis X is characterized by a proliferation of cytologically benign histiocytes and occurs in a severe form (type 1) with a short course and poor prognosis and a milder form (type 2) with a protracted course and favorable prognosis. Otolaryngologists will encounter the type 2 form of the disease and must include it in the differential diagnosis of osteolytic lesions of the skull.