Osteomas occur throughout the temporal bone and, depending on their location, may cause tinnitus, hearing loss, vertigo, and facial nerve paresis. We present a rare case of a 25-year-old woman with a mastoid osteoma enlarging over a 6-month period. Other than a cosmetic deformity of her upper neck, the patient was asymptomatic. After surgical removal of the bony neoplasm, the patient was noted to have a high-frequency sensorineural hearing loss. This case study presents clinical, radiologic, intraoperative, pathologic, and audiometric findings of a mastoid osteoma and a review of the literature.