A 17-year-old boy with excessive hypernatremia, hyperchloremia and hyperosmolality caused by an intra- and suprasellar germinoma was studied. Destruction of hypothalamic thirst and osmoregulation centers were responsible for these disturbances. The extra- and intracellular space were reduced, the exchangeable sodium was 2.5 times normal. Urinary gravity was 1,022 following dehydration, indicating that the patient could produce endogenous vasopressin. The syndrome was accompanied by normal urinary excretion of aldosterone in the presence of a high plasma renin concentration (PRC) of 4.5 X 10–4 GU/ml. There was a partial impairment of pituitary function for the release of HGH and possibly also for ACTH.