The sensorineural component of a mixed hearing loss due to otosclerosis is generally accepted as due to the otosclerotic lesion. The existence of pure cochlear otosclerosis without stapes fixation has been questioned. However, we are documenting 7 such cases in a separate publication. Results of this study, which evaluates 46 temporal bones with clinical or cochlear otosclerosis, demonstrate that the degree of sensorineural loss is directly related to the amount of hyalinization of the spiral ligament. The hyalinization occurs adjacent to active otospongiotic lesions but not next to inactive otosclerotic lesions. Both types of lesions may involve the cochlear endosteum. Small channels through the endosteal bone from the lesion to the spiral ligament have been found. The hyalinization spreads laterally from these channels. The hyalinization is presumably a result of the passage of toxic substances (proteolytic enzymes) from the lesion to the ligament. Strial atrophy is most pronounced on ligaments with the greatest degree of hyalinization. Hyalinization in only one ear produces decreased hearing compared to the other ear.