Otosclerosis is a condition characterized by bony changes involving the labyrinth capsule and leading in many cases to fixation of the stapes in the oval window. Broadly speaking, the change is one in which the dense bone of the normal labyrinth capsule is transformed into swollen vascular osteoid tissue which encroaches on the annular ligament and footplate of the stapes leading to ankylosis. This new vascular bone is sharply defined from the normal bone and stains deeply with basic dyes. It is confined to localized areas, the most usual site being the promontory between the two windows. This region is one in which vessels enter the labyrinth wall and in which cartilage "rests" are frequently found in early life, besides being subjected to stresses not felt by other parts. Foci are found in other parts of the labyrinth capsule remote from the mucosa and even in the internal auditory meatus.