A study of 164 temporal bones with otosclerosis revealed stapes fixation in 123. Large lesions (4 mm or larger) were characterized by involvement of the cochlear endosteum, increased cellular activity, and bone resorption. Anatomical sites of predilection were: 1. anterior to the oval window, 2. margins of the round window, and 3. apical-medial wall of the cochlea. Two of the five ears with cochlear otosclerosis were from a patient with severe bilateral hearing loss. The size, cellular activity, and location of otosclerotic lesions showed no correlation with the magnitude of sensorineural hearing loss. Subjects over 69 years of age had greater bone-conduction losses than age-matched controls. Nine ears showed occlusion of the round window niche.