The most characteristic histology of the cholesteatma is the proliferation of the squamous cell lining of the lesion. The origin of the cholesteatoma in the middle ear is controversial because there is no squamous cell lining in the normal condition. The squamous cell lining of the cholesteatoma I of either the keratinizing or the nonkeratinizing type with a variable histologic maturity. Based on these findings, to study the a variable expression of the cytokeratins, the immunohistochemical staining of the cytokeratin(CK 10, CK 13, CK 18) on the squamous epithelium of the cholesteatoma was analyzed. At histopathological findings, the epithelium of the cholesteatoma composed of the keratinized stratified squamous epithelium in 89.8% and the non-keratinized stratified squamous epithelium in the remainder. These epithelia showed a transitional zone and the glands, which are not found in normal middle ear mucosa. CK 10 is expressed in the upper layer of the keratinizing epithelium : CK 13, in the whole layer of nonkeratinizing epithelium : and CK 18 in the glandular epithelium. These findings show that both CK 10(expressed at epidermis) and CK 13(expressed at middle ear mucosa) are expressed at the epithelium in the cholesteatoma. And the facts that the epithelium of the cholesteatoma is mostly of the keratinizing type and that the focal expression of CK 10 & 13 occurs in the transitional zone suggest that the epithelium of the cholesteatoma is differentiating from the nonkeratinizing toward the keratinizing squamous epithelium. Finally, it was suggested that the epithelium of the cholesteatoma was continually proliferating and differentiating toward the keratinizing squamous epithelium.