During the period of last 10 years from 1964 to 1973, a clinical observation was made on 9 cases under the impression of post-operative maxillary cyst which treated in Dept. of otorhinolaryngology, National Medical Center. All of the cases complained of cheek swelling with discomfort, nasal obstruction and headache. During operative prodecure, yellowish-brown colored fluid in cyst were noted in 7 cases and bony destruction were confirmed in 5 cases mainly at the anterior and medial wall of maxillary sinus. The above all cysts were developed after intial Caldwell Luc's operation, especially a cyst among the 9 cases was reappeared on affected side following second operation for post-operative maxillary cyst.