Hyponatremia is a common electrolyte disorder encountered in hospitalized patients. A preliminary, observational, feasibility analysis finalized to assess retrospectively the incidence of hyponatremia (Serum sodium < 135 mEq/L) in a general medical-surgical hospital and the distribution of the cases of hyponatremia among different referral units showed that over one year observation there were more than 1500 cases of hyponatremia. Conventional therapy for hyponatremia depends on its causes, speed of onset, extracellular fluid volume status, and severity. Treatment consists in fluid restriction, normal or hypertonic saline, furosemide. Recent development of arginine vasopressin antagonists has provided a new therapeutic option for treatment of hyponatremia.Tolvaptan, an orally administered, nonpeptide, selective vasopressin V2 receptor antagonist reported to increase free water clearance and limit fluid retention in subjects with congestive heart failure or liver cirrhosis, has been also shown to be effective in the treatment of chronic hyponatremia in patients with SIADH, chronic heart failure, liver cirrhosis. Thus the investigators designed a clinical study to explore the incidence of severe hyponatremia in hospitalized patients in the setting of large general hospital and to evaluate whether tolvaptan is effective and safe in increasing serum sodium concentration in patients with normovolemic and hypervolemic hyponatremia in the setting of daily clinical practice. Moreover this study may help understand the cost-effectiveness of tolvaptan therapy compared to traditional treatments of hyponatremia.