In hypotonic hyponatremia, which is the most common form of hyponatremia, clinical manifestations are largely due to brain swelling caused by entry of water into the cells. In acute and severe hyponatremia, dramatic symptoms, such as seizures, acute psychosis, permanent brain damage, brain-stem herniation, leading to coma and death may occur. In chronic hyponatremia, symptoms are generally less dramatic and may include headache, nausea, vomiting, gait alterations, muscle cramps, restlessness, and disorientation. It has become evident in recent years that mild forms of chronic hyponatremia may also be associated with clinical signs, if carefully investigated. Several studies also reported an increased length of stay in the hospital of patients with hyponatremia in different clinical settings, leading to increased costs. Most important, this condition has been clearly associated with a significantly increased risk of death, even when serum [Na+] is slightly reduced. On the contrary, there is convincing evidence that the mortality risk is reduced when hyponatremia improves.