Hyponatremia due to intolerance to water is a frequent clinical condition and associated with increased mortality. Besides the well known neurological symptoms, gait disturbances, falls, fractures and osteoporosis have also been described recently in patients with chronic hyponatremia. Acute hyponatremia is a more dramatic situation and needs rapid action when severe neurological symptoms are present. Hypertonic saline is recommended to treat this condition until relief of severe symptoms. The causes of hyponatremia have to be carefully examined. Especially diuretics, antidepressants and endocrine causes, e.g. hypothyroidism, hypocortisolism and hypoaldosteronism should be excluded by examination of the patient history, clinical examination and by laboratory tests. Patients should be classified as being euvolemic, hypovolemic or hypervolemic. Whereas acute hyponatremia with severe symptom should be treated with hypertonic saline, euvolemic hyponatremia due to the syndrome of inappropriate antidiuretic hormone secretion (SIADH) with mild and moderate symptoms can now be treated with tolvaptan, a selective V(2)-vasopressin antagonist. Oral tolvaptan has been shown to be an effective and potent aquaretic to treat hyponatremia caused by SIADH as evidenced by a simultaneous increase in serum sodium and a decrease in urine osmolality. The condition of patients with mild or moderate hyponatremia is also improved. Side effects associated with tolvaptan include increased thirst, dry mouth, polyuria and hypernatremia. Rapid increases in serum sodium should be avoided by close monitoring in a hospital setting.