Statistical analyses of histopathologic findings in the cochlea and spiral ganglion of 37 temporal bones with otosclerosis, 12 controls of similar age, and seven controls with normal hearing were performed. In temporal bones with otosclerosis there was significant atrophy of the spiral ligament and stria vascularis in regions with endosteal involvement by otosclerosis, compared with regions without endosteal involvement (P less than .0001). There was more generalized atrophy of the stria vascularis in cochleae with two or more sites of endosteal involvement by otosclerosis than in cochleae with only one site of endosteal involvement (P less than .02), cochleae in temporal bones with otosclerosis but without endosteal involvement (P less than .05), or cochleae of controls of similar age (P less than .007). In addition, there was more atrophy of the spiral ligament in cochleae with two or more sites of endosteal involvement than in cochleae of similar age from the control group (P less than .03). In temporal bones with otosclerosis, there was no significant difference in counts of outer hair cells and density of spiral ganglion cells between regions demonstrating endosteal involvement by otosclerosis and regions without such involvement. However, total outer hair cell counts were lower in cochleae with two or more sites of endosteal involvement of otosclerosis than in cochleae with one site of endosteal involvement (P less than .04), cochleae in temporal bones with otosclerosis but without endosteal involvement (P less than .02), or cochleae from individuals of similar age but without otosclerosis (P = .05). Comparison of the mean bone conduction threshold, as measured in life, in temporal bones with otosclerosis compared with the air conduction threshold in aged-matched controls, demonstrated that only cochleae with two or more sites of endosteal involvement had a mild but statistically significant (P = .05) decrease in hearing. There was no evidence to support the concept that otosclerotic foci without stapedial fixation frequently cause significant degeneration of the cochlea or elevation of bone conduction thresholds.