Hyponatremia is the most frequent electrolyte abnormality seen postoperatively in pediatric patients receiving maintenance fluid therapy. Hyponatremia is also common in acute pediatric illness. The main factors contributing to hyponatremia in these conditions are increased secretion of antidiuretic hormone (ADH) and routine use of sodium hypotonic fluids. An increased ADH secretion results in an impaired ability to excrete free water. If the sodium concentration falls to less than 125 mmol/L hyponatremic encephalopathy might develop, resulting in cerebral edema. This is avoided if hypotonic maintenance fluids are not used perioperatively or for rehydration or maintenance during acute critical illness in children.