SUMMARY　 Abnormality of sodium metabolism is commonly encountered in in-patients. Incidence of hyponatremia is about 23% of in-patients. The prognosis of patients with hyponatremia is poorer that that without hyponatremia, in general hospital, ICU and CCU. Hyponatremia itself induces hyponatremic encepharopathy, however, rapid correction of hyponatremia also induces osmotic demyelination. Therefore, correction of serum sodium concentration should be started gently in patients with neurological symptoms, such as irritation or coma. Maximum correction of sodium concentration should be less than 12 mEq/L in 24 hours. To differentiate the cause of hyponatremia, 1) measurement of plasma and urine osmolarity, 2) serum potassium and urine sodium concentration, 3) volume status, 4) special hormone measurement 　　Hypernatremia is also common and related with poor prognosis. Rapid correction of hypernatremia also induces osmotic demyelination.