Infected mucocele is a characteristic cystic swelling of the paranasal sinus which contains mucopurulent secretion. The cyst occupies the whole of one sinus which can make progressive enlargement and dissolution of the thin interventing bony wall. Finally the bony wall becomes thin and disrupts so that the cyst may displace the neighboring organs. The authors recently experienced three cases of infected mucocele of the paranasal sinuses. All of them were treated with surgical procedure which was removal of mocous membrane as entirely as possible and creation of wide naso-frontal duct or counter opening. The postoperative courses were satisfactory.