The postoperative occurrences of mucocoeles of the maxillary sinuses are much rarer than those of the frontal or ethmoid sinuses. Their occurrence following Caldwell-Luc operations is 1:150. They may originate from obliteration of the sinus ostia (antritis dilatans) as well as from residuel mucosa in the upper lateral recesses of the sinus cavities. There are four clinical types of mucocoeles: (1) those which present with a swelling of the cheek, (2) those with destruction of the orbital floor with elevation of the eye, (3) those with destruction of the hard palate (which are extremely rare), and (4) those which present as multiple cysts (also rare). Among the author's 20 cases, there were 15 of the first group, 4 of the second and one of the fourth. In differential diagnosis, ameloblastoma of the maxilla must be considered. When suppuration (or pyocoele) occurs, the clinical picture may be similar to that of periorbital edema or inflammation. Operative treatment requires complete removal of the mucocoele and restoration of nasal drainage. When destruction of the orbital floor has occurred, reconstruction with a free septal cartilage graft is recommended.