Hyponatremia is a frequent occurrence after pituitary surgery, having been described in 9% to 35% of the patients. It is produced by the syndrome of inappropriate secretion of antidiuretic hormone (SIADH) or, more frequently, by the cerebral salt-wasting syndrome (CSWS). The clinical presentation of both syndromes is identical and the differential diagnosis can be difficult. The determination of the volemic state is essential for the diagnosis, since the patients with the SIADH are euvolemic or hypervolemic, while those with CSWS are hypovolemic. Several methods can be used to detect the volemic state in these patients and, among them, the furosemide test can best discriminate between SIADH and CSWS. A furosemide infusion (20 mg) normalizes sodium serum levels in SIADH patients, but not in CSWS patients who remain hyponatremic. The differentiation between the 2 syndromes is clinically relevant since their treatment is antithetical. SIADH patients need liquid restriction of liquids and/or furosemide to reduce the volume of extracellular water, while CSWS patients need volume replacement with sodium supplementation (or fludrocortisone can be a good alternative). The diagnosis and treatment of these syndromes are discussed on the basis of the literature reports.