It is held that cysts in jaws enlarge as the cyst fluids increase and that the intracystic fluid pressure on the jaw bones increases because of inflammation. According to this investigation, cysts enlarge in the following manner: when a cyst is infected, the cyst wall becomes almost bloodless; cell membranes of the wall are destroyed, and unsaturated fatty acids depart from the membranes; these unsaturated fatty acids generate prostaglandins and lipoperoxides which lower the reaction of cyclic AMP; the result is resorption of the jawbone; therefore, control of the cyst inflammation is the first requirement in the treatment of the cyst. For control of the inflammation, exchanges of cyst fluids for liquids, including antibiotics, were carried out once a week. As a result, the volume of cyst fluids decreased, the cysts diminished in size, and clearing tissues were scarcely seen. It can be maintained that this irrigational method is effective for the treatment of cysts in jaws.