We have developed a unique method of quantitative computed tomography (QCT) that enables measurement of the density of the cochlear capsule in vivo. We performed pure-tone audiometry and QCT on 67 ears from 35 subjects with radiographically confirmed Paget's disease of the skull and on 40 ears from twenty volunteer subjects. The Pearson product-moment correlation coefficients (age- and sex-adjusted) in the group affected by Paget's disease were -0.63 for left ears and -0.73 for right ears for high-frequency air conduction pure-tone thresholds (mean of 1, 2, and 4 kHz) versus cochlear capsule density. Correlation coefficients (age- and sex-adjusted) between cochlear capsule density and air-bone gap (mean at 0.5 and 1 kHz) for the affected group were -0.67 for left ears and -0.63 for right ears. All correlations between hearing thresholds and cochlear capsule density in pagetic subjects were significant at p < 0.001. The regressions were consistent throughout the ranges of hearing level. There were no significant correlations between cochlear capsule mean density and hearing level in the volunteer subjects. These findings demonstrate the feasibility of precise and accurate density measurements in the temporal bone in vivo and support the use of the mean cochlear capsule density as a marker of disease effect. Alteration of cochlear capsule bone density may be related to the mechanisms of hearing loss in Paget's disease of bone.