Despite its innocent histopathologic apprearance, cholesteatoma may follow an aggressive clinical course with extensive bone destruction leading to deafness, vertigo and meningeal and brain involvement. The assessment of the pathogenesis of the cholesteatomas in human subject is limited because they are generally not recognized until symptoms occur, that is at a later stage of the disease. There have been several studies using experimental animal models to clarify the characteristics of human cholesteatoma. In the present study, an autologous free skin graft taken from the auricle was transplanted in the otic bulla of 16 normal adult mongolian gerbils. In 8 of them, the graft was placed directly over the intact mucosa(group 1). In others, it was placed on the exposed bone following removal of the mucous membrane(group 2). After completion of these procedures, mongolian gerbils were sacrificed three months later for histological study. 1) After three months, epidermal cyst simulating cholesteatoma was produced in 58% over all. In group, 1, it was produced with 50%, and in group 2 it was produced with 64.3%. 2) Accumulation of keratin debris inside the cyst and subepithelial inflammatory cells were noted. No histological difference between group 1 and group 2 could be seen, but group 2 seemed to have more subepithelial inflammatory cells.