The bony form of eosinophil granuloma in histocytosis X, raises diagnostic and prognostic problems for the surgeon. 15 cases are reported here which permitted the authors to perform a rapid general review of this disease and judge its course as 10 patients were followed up with a minimum follow up period of 4 years. These observations confirm: --that the local course is always spontaneously favourable: --that the risk of a passage from an extra-bony form of histiocytosis X, should be considered all the more serious when the subject is younger, when there exist certain localisations, such as the mastoid, where there are certain general clinical signs or biological disturbances. Apart from exceptional symptomatic treatment, such as spinal decompression or treatment of a spontaneous fracture, the essential stage of clinical examination is biopsy. Medical treatments do not seem to have any influence on the bony lesion and are not without risk (radiotherapy). Although depending on various criteria, the prognosis should always be reserved and the patient should be followed up for a long period.