The Caldwell-Luc's operation is one of the most common operating procedures of chronic maxillary sinusitis in Korea. There were many post operative check cysts reported as a latent complication with in several years or even far more years later. We have treated 25 cases of post operative cheek cyst during the past 5 years from 1969. 6 to 1974. 6. It is to report the etiology & prevention of post-operative cheek cyst with the review of literatures. Upon reviewing literatures with the clinical experences the following results were noted. 1) Post-operative cheek cyst may occur regards sex, the age of prevalence ranging from 30 to 50; and this complication must be distinguished from other tumor of the paranasal sinuses. 2) Post-operative cheek cyst usually appears as a form of retention cyst. As to its etiology the auther has an opinion that operative wound may chang in the process of healing so as to form cyst. 3) The author esteems it most important for the prevention of the complication that maxillary sinus should be kept clean, leaving no mucous membrane when the site is operated and that obstruction of pathway to ethmoid sinus must be prevented by enlargement of natural opening and counter opening.