Hyponatremia is a state of relatively common observation among neurosurgical patients and it may determine or precipitate a condition of brain swelling in the head-injured. The quick reversal to normal values of serum sodium concentration may cause relevant neurological disturbances: they are related to peculiar neuropathological changes, the most representative of which is central pontine myelonilysis. Two cases (one in pediatric age) of head injured patients with brain damage resulting from rapid correction of a SIADH related hyponatremia are presented.