The goal of this temporal bone study was to quantify the relationship between specific histologic changes at the stapes footplate and the magnitude of the air-bone gap in otosclerosis. The study material comprised 26 specimens with otosclerosis and 37 age-matched controls. Detailed anatomic measurements were made on each histologic section through the stapes footplate in each bone, resulting in 30 different measurement parameters for each bone. For frequencies 250 to 2,000 Hz, the conductive hearing loss correlated highly with (p <.01) and appeared to be caused primarily by narrowing and loss of the annular ligament, especially at the posterior stapediovestibular joint space. The size of the air-bone gap appeared to be determined by the extent and degree of this pathologic change. Schuknecht's hypothesis that bony ankylosis of the footplate would be associated with an air-bone gap of >30 dB was supported by our data. However, the degree and extent of bony footplate ankylosis could not be reliably predicted by the size of the air-bone gap.