The detailed mechanism of bone resorption in cholesteatoma was investigated by means of eroded ossicles obtained during middle ear surgery for cholesteatoma. In the light microscopic study, multinucleate osteoclasts with ruffled borders were found in contact with the eroded bone, which appeared to be osteoclastic lacunae. Scanning electron microscopy revealed the lacunae to be many absorption bays, 30 to 100 microns in diameter, clustered on the surface of eroded areas. Numerous cone-shaped stubs of digested collagen fiber bundles, consisting of scores of collagen fibrils and degenerated extracellular organic substances, were visible at higher magnification on the bottom of the absorption bay. The pattern of fusion and twining of the disarranged collagen fibrils at the top of the partly digested fiber bundles was clearly rendered by the alkali-water maceration method for scanning electron microscopy. We infer from the morphological evidence that osteoclastic resorption may be one of the major mechanisms of bone destruction in cholesteatoma, and that demineralization and degeneration of extracellular organic substances precede disruption of collagen fibrils at the front of bone resorption.