A four-year-old girl suffered difficult-to-diagnose hyponatremia resistant to treatment following surgery for a suprasellar tumor. The final diagnosis was diabetes insipidus evolving in three stages. Hyponatremia is a common problem following surgery to remove brain tumors. Early diagnosis and treatment of this electrolytic imbalance are essential for preventing serious neurological symptoms or death. The conditions most closely related to hyponatremia are inappropriate antidiuretic hormone secretion syndrome (IADHSS) and cerebral salt wasting syndrome (CSWS). The latter has become more common in recent years among patients undergoing brain surgery. Whereas IADHSS is treated by restricting fluids, CSWS requires administration of salt and volume fluid volume. We believe that for differential diagnosis of postoperative hyponatremia, a fluid restriction test takes priority over of fluid loading following neurosurgery. The course of hyponatremia must be carefully monitored and a complete endocrinological workup must be performed to detect the possible presence of hypophyseal deficiencies, particularly hypothyroidism and suprarenal insufficiency.