ME NINGIOMAS of the posterior cranial fossa are difficult to diagnose but, when detected, are often amenable to surgical removal. The clinical, roentgen, and surgical features of these lesions have been ably described in the comprehensive treatise by Castellano and Ruggiero in 1953 (5) and in other reports (4, 6-8, 1518,21,26,34,39- 42). It is the purpose of this article to present the findings observed in patients at the Graduate Hospital and the Hospital of the University of Pennsylvania from the year 1918 through 1955. The study is based on the records of 59 of 64 patients, most of whom were operated upon by Dr. Francis Grunt3 and Dr. Robert Groff.4 Radiographs were available for review in only 26 cases; in the others the radiologic interpretations were consulted. Incidence This series represents approximately 1 in 12 of all patients with verified intracranial meningiomas operated upon by Grant and Groff, a proportion similar to that reported by Cushing and Eisenhardt (7), Dandy (see Gonzalez Revilla, 14a) Horrax (21), and Campbell and Whitfield (4). The ages of the patients at the time of the diagnosis of posterior fossa menin gioma were as follows: The 71 cases reported by Castellano and Ruggiero constituted 8.45 per cent of proved meningiomas from the Serafimer Hospital, Stockholm (803 cases or 19.2 per cent of a total of 4,185 verified intracranial brain tumors). Gliomas are the commonest neoplasm occurring in the posterior fossa. The acoustic neurinoma is the second in frequency, and meningiomas the third. The ra tio of neurinoma to meningioma has been variously given as 6 to 1 (5), 9 to 1 (4), 10 to 1 (18), and 15 to 1 (39). Classification and Distribution While posterior fossa meningiomas have been classified by different authorities in from two to five major groups, we have adopted the classification offered by Castellano and Ruggiero, based on the method for classifying supratentorial meningiomas, by the site of dural origin or attachment. The posterior fossa meningiomas arise in the following sites: Class I, cerebellar convexity; Class II, tentorium cerebelli; Class III, posterior surface of the petrous portion of the temporal bone (meningiomas of the cerebellopontine angle); Class IV, clivus; Class V, foramen magnum. Among our patients were some in whom it was extremely difficult or even impossible to tell from which portion of the dura in the posterior fossa the meningioma arose. Occasionally, the operative report failed to mention specifically the site of attachment. The number of cases in each of the categories outlined above, and described in greater detail below, is shown in Table 1. The illustrative material presented was selected primarily to demonstrate those diagnostic points that proved significant in the early recognition of the tumor.