Lithium is a widely used important drug in the treatment of manic-depressive illness and its prevention of relapse. However, this drug has a Low therapeutic index, therefore, it has many attendant side effects. The most prevalent renal effect of lithium is impairment of concentrating ability and this defect appeared into overt polyuria. A renal lesion is confined to the collecting tubule and 12-20% of patients taking lithium suffer from nephrogenic diabetes insipidus. This nephrogenic diabetes insipidus causes the states of extracellular fluid depletion, hypernatremia and precipitates lithium intoxication. In such situation, symptoms of nephrogenic diabetes insipidus and lithium intoxication are very similar, so we should be very cautious to discriminate them. We herein report a patient characterized by a prolonged stuporous state, hypernatremia and severe nephrogenic diabetes insipidus during lithium therapy.