I observed 252 postoperative maxillary cysts clinically and with the electron microscope. In 49 of the cases, the cysts were in the immediate proximity of teeth. There were three categories of cysts: type I, where the root was in the cyst cavity, type II, where the cyst wall was fused to the tooth, and type III, where the socked extended to the cyst cavity. Type I is referred to as the postoperative radicular cyst. Eighty of the 252 cysts were observed by scan-ning electron microscopy, and these were divided into five types based on the number of ciliated epithelial cells. Al-though ciliated epithelial cells were observed in 57 cysts, there were large variations in the number of cilia. Although the onset and development of the unilocular cyst might be explained by the sinus obstruction theory, this is not the case for multilocular cysts.