Objectives: To investigate the patterns of cytokeratin (CK) expression in retraction pocket cholesteatoma. Study Design: An animal model study. Methods: Retraction pocket cholesteatomas were induced by electrocautery of the eustachian tube orifice in 24 mongolian gerbils. They were divided into normal and cholesteatoma groups of clinical stages I to IV. The antibodies to pan-cytokeratin CK 1/10, CK 5/6, CK 4, and CK 13/16 were used for immunohistochemical staining. The intensity of staining in each group as measured with densitometry was compared regarding anatomical sites and clinical stages. Results: In retraction pocket cholesteatoma, CK expression was altered only at focal sites such as the pars tensa of the tympanic membrane. The change of CK expression was observed only at certain stages of cholesteatoma formation. In keratinocytes from cholesteatomas, CK 13/16 was overexpressed compared with control specimens, indicating hyperproliferation. The site with the most prominent change in retraction pocket cholesteatoma was somewhat different from that in canal ligation cholesteatoma in a previous study. Conclusions: The results suggested that aural cholesteatoma is a disease with a spectrum of pathological conditions and that the transmigration and hyperproliferation process of squamous epithelium occurs in areas adjacent to the cholesteatoma.