Of the bony tumors of the skull, those of the external auditory canal and of the accessory nasal sinuses are perhaps the most commonly encountered by the otolaryngologist. Osteoma of the mastoid process is infrequent in occurrence; Haymann1in 1919 was able to find but 8 authentic cases. Since that time, reports by Frankel,2Bruzzi3and Breyre4have appeared. Because of this dearth of clinical material and the absence of any comparable data in the American literature, it was deemed advisable to record the following case. REPORT OF A CASE M. S., a Negress aged 52, came to the otolaryngologic clinic at Cook County Hospital early in December 1937 because of a hard mass behind the right ear, which had been slowly increasing in size for ten years (fig. 1). The patient recalled a severe blow by a fist on the involved area approximately two years prior to the initial