Primary purpose of the lateral nasal window of inferior meatus in Caldwell-Luc operation is to promote overflow drainage postoperatively into the nose [1,2]. This lateral window,however,often narrows and even closes after surgery. This narrowing of the window has been considered to be one of the causable factors for postoperative maxillary sinus cyst. In our clinic,either narrowing or closure of the lateral nasal window of inferior meatus was found in 69% of the total patients with postoperative maxillary sinus cyst.