DURING the past 25 years there has developed an increasing interest among neuropathologists in primary intracranial sarcomas. There has, in general, been agreement concerning the diagnosis of diffuse sarcomatosis of the meninges, melanomatosis of the meninges, and meningeal and intracerebral fibrosarcoma. However, the interpretation of those types of sarcoma which Bailey1described in 1929 as perithelial and alveolar has been extremely varied, and at present several different concepts exist. From our clinical and pathological study of 13 cases that have been diagnosed as perithelial sarcoma in the Montreal Neurological Institute since 1936, we have concluded that the types previously termed perithelial and alveolar probably represent variations of a single entity. It is with this neoplasm, which we have continued to designate by the well-established term perithelial sarcoma, that this report is concerned. REVIEW OF LITERATURE An increasing number of reports* of primary intracranial sarcomas have appeared during recent years,