Malignant hyperthermia, first described by Denborough (1962), is rare disease which is occurred accidentally and rapidly aggrevated during or after anesthesia. The mortality rate is very high because rapid and abrupt elevation of idiopathic faver is caused remarkable changes of various metabolism and rapid deterioration of physical status. Etiology of this syndrome was not proved, but hereditary tendency, autosoml dominant gene, was suspected in combination with abnormality of heat metabolism, decrement of A.T.P. and elevation of creatine phosphokinase. The treatment of this syndrome is not specific and followings are available ; 1) rapid elimination of anesthetics, 2) elimination of muscle relaxants, 3) cessation of operation, 4) decreasing of body temperature, 5) correction of acidosis. Recently the author experienced a case of malignant hyperthermia during removal of thyroglossal duct cyst under general endotracheal anesthesia.