The bone mineral density of the internal auditory meatus was investigated by means of quantitative computed tomography (QCT) in 12 patients with acoustic tumor. Investigated portions of the internal auditory meatus were the anterior and posterior porus and the anterior and posterior fundus. The bony vestibule and cortical bone of the mastoid cavity were also investigated. The bone density values (calcium carbonate equivalent value) for each portion were analyzed statistically. Another investigation by CT program "Profile" revealed changes in CT values along the posterior wall of the internal auditory meatus, and the results were compared with that of the QCT. The following results were obtained: 1) The highest calcium carbonate equivalent value was found in the posterior fundus, the lowest in the anterior porus. There were significant differences between the bilateral fundus and porus values. 2) There was a marked difference only in the anterior fundus of the abnormal side in comparison with the normal side, while differences in the other 5 portions were not statistically significant. 3) There were no significant differences in any of the portions between cases of superior vestibular nerve origin and those of inferior vestibular nerve origin. 4) In Profile, the CT values increased gradually from the porus to the fundus on both sides. A dip formation, indicating a lower CT value, was found between the porus and fundus in many cases. It was suggested that this dip formation was influenced by pneumatization of the posterior wall of the internal auditory meatus.