Middle ear inflammation by inflammatory agents & insufficient ventilation by tubal occlusion inhibits the mastoid pneumatization in experimental animals. Suppressed mastoid pneumatization in chronic otitis media & cholesteatoma are common findings in human. Clinically, it has been observed that huge cholesteatoma had a large mastoid cavity and attic or atticoantral cholesteatoma had small mastoid cavities in mastoid surgery. Authors observed the effects of mastoid pneumatization & sclerosis on the progression of cholesteatomas by analysing the mastoid size of cholesteatomas on temporal bone CT and suggested the possible pathogenesis of cholesteatoma. The results were as follows : 1) Average age was 27.8 years in huge cholesteatomas, but 35.4 years in atticoantral cholesteatoma. 2) Mean area of mastoid in control, huge cholesteatoma, atticoantral cholesteatoma, and attic cholesteatoma were 8.96 cm², 6.86 cm², 4.82 cm², 4.97 cm², and areal ratios of both mastoids were 92%, 98%, 74%, and 80%. 3) Areal ratios of cholesteatoma side of huge, atticoantral and attic cholesteatoma to control were 77%, 54% and 56%. The results suggested that mastoid of huge cholesteatoma was larger than those of antral and attic cholesteatoma. So, the huge cholesteatoma may destroy the mastoid after pneumatization, and mastoid sclerosis which is induced by middle ear inflammation, may prevent the bone destruction.