Hyponatremia is defined as plasma concentration of sodium lower than 135 mmol/L. It usually does not reflect a true sodium deficiency, but rather free water retention caused by vasopressin hypersecretion, with hypoosmolality of body fluids. Hyponatremia may be caused by different diseases and pathological conditions, such as: hypovolemia, hypothyroidism, adrenal insufficiency, syndrome of inappropriate antidiuretic hormone secretion (SIADH), congestive heart failure, hepatic cirrhosis, and adverse drug reactions. Neurological symptoms of hyponatremia result from brain edema, and depend on the rate of sodium concentration decrease and degree of the disorder. The treatment includes elimination of free water, most often through volume expansion, water restriction in SIADH or deficient hormones supplementation. The rate of correction of sodium concentration in chronic and profound hyponatremia should not exceed 10 mmol/L during the first 24 hours, and 18 mmol/L during the first 48 hours. Overly rapid correction of natremia may result in irreversible demyelinating damage of the central nervous system.