Desmopressin, a synthetic analogue of the antidiuretic hormone, is an effective medication for primary nocturnal enuresis for both children and adults. Its safety is well established. Although it has a favorable side effect profile, because of its pharmacological effect, intranasal desmopressin can rarely induce water intoxication with profound hyponatremia if given without adequate restriction of water intake. The authors describe an adult patient with water intoxication and severe hyponatremia accompanied by loss of consciousness and seizures after 2-day intranasal administration of desmopressin. The present and the previously reported cases emphasize the need for greater awareness of the development of this serious and potentiallyfatal complication. In addition, to adjust the drug to the lowest required dosage, adequate restriction of water intake is recommended, and serum levels of sodium should be measured periodically to allow for early detection of water intoxication and hyponatremia.