Mucocele of the accessory nasal sinuses has been regarded with interest for many years. Though it has long been considered a rarity, the gradually increasing number of reports in the literature demonstrates that it is more common than is generally supposed. It seems important, therefore, that its nature and clinical manifestations should be thoroughly understood if mistakes in diagnosis and treatment are to be avoided. By the term mucocele is understood the accumulation and retention within a sinus of a mucoid secretion owing to continuous or periodic closure of its ostium and resulting in gradual thinning, distention or actual erosion of one or more of its walls. The frontal and anterior ethmoid cells are most commonly affected. Mucocele of the sphenoidal sinus is extremely rare; in fact its occurrence is disputed by many. Cases, however, have been reported by such careful observers as Hajek1 and Gerber. While mucocele