A case of retromaxillary cholesteatoma mimicking maxillary cyst Cholesteatoma refers to mass formation by keratin layers inside the hyper-keratinized squamous epithelium that partially or entirely replaced the normal mucous membrane. Even though cholesteatoma inside the paranasal sinus is extremely rare compared to that in the tympanum, It has been reported a few cases especially in frontal sinus. However, retromaxillary cholesteatoma is even less common than sinus cholesteatoma and has been reported only little so far. Recently we experienced a case of retromaxillary cholesteatoma in 18-year-old female patient. She visited our clinic for a left maxillary sinus mass which was unintentionally detected in the Brain CT. We considered the possibility of maxillary cyst and performed endoscopic operation. In the operation, a mass containing some keratin materials was removed, which was originating from the retromaxillary region and pushing the posterior maxillary wall. Finally, the mass was diagnosed as a cholesteatoma histopatholgically. The case has been reported herein together with literature review as a case of left retromaxillary-originated cholesteatoma.