Two male newborns developed severe life threatening hypernatremia with serum sodium levels of 181 and 196 meq/l respectively. Both children were fed a two-to fivefold concentrated powdered infant formula since birth. Shortly before admission diarrhea developed in both patients. The children were admitted during the third week and fourth week respectively, of their lifes with clinical signs of severe hyperosmolar dehydration. In both, treatment of shock was instituted and was followed by rehydration therapy. Great care was taken to lower serum sodium by not more than 15 meq/l/day in order to prevent cerebral edema. Inspite of these measures, the first patient developed cerebral seizures 5 hours after initiation of fluid replacement. The patient was intubated, and measures aimed at prevention of cerebral edema were started (hyperventilation). The eventual outcome was satisfactory, on follow up the patient showed no signs of persistent cerebral damage. In the second patient the same management was started from the beginning and no cerebral problems occurred. Restoration of stable body functions with a normal electrolyte status was achieved within five days.