An experimental study was conducted in chinchillas regarding the pathogenesis of acquired cholesteatoma (keratoma). The placement of a chemically modified gelatin membrane from the external auditory canal to the promontory through a tympanic membrane perforation stimulated squamous epithelial cell migration. Cholesteatoma formation with the presence of keratin debris and inflammatory reactions was observed in the middle ear and anterior bulla in 53.5% of the experimental animals. These experimental findings show for the first time the presence of epithelial migration and true cholesteatoma formation in the middle ear of chinchillas in an experimental model with deliberate perforation of the tympanic membrane. Erosion of the cochlear walls was observed in areas with granulation tissue and cholesteatoma. The importance and significance of the migration of squamous epithelium and of the middle ear inflammatory reaction in the genesis of acquired cholesteatomas are discussed.