Clinical observations were performed on 100 patients who were treated for postoperative maxillary cyst during the seven year period from April, 1969 until March, 1976. The following results were obtained.1. The data on patients' sex, affected side, age of onset, onset-postoperative period relation, nasal findings and x-ray findings were similar to those of some other reports.2. Postoperative maxillary cyst was frequently observed after the Caldwell-Luc operation.3. The cyst developed more frequently when maxillary sinus was only operoted at the primary operation.4. In some patients, their teeth were involved in cysts. They should be called postoperative radicular cyst.5. Postoperative maxillary cyst is caused by the closure of the natural ostium and intranasal opening which was made at the primary operation in the inferior meatus.6. In preventing postoperative maxillary cyst, the following five should be kept in mind:1) complete resection of the sinus mucosa: 2) minimal operative bleeding: 3) selection of the operative method (Watsuji-Denker method is more preferable): 4) prevention of the closure of the natural ostium: and 5) treatment of the teeth exposed to the sinus. Among these, the prevention of the closure of the natural ostium is most important.