Hyponatremia is a well known complication of traumatic and nontraumatic cerebral injury, often related to the syndrome of inappropriate antidiuretic hormone secretion (SIADH). Nonetheless, it also can be associated with a different entity, the syndrome of cerebral salt wasting (CSW). The authors report the case of a 4.5-year-old boy presenting with major head injury who at day 6 after admission had generalized tonic-clonic seizures caused by severe acute hyponatremia (serum sodium level, 119 mmol/L) and signs of dehydration. Despite initial isotonic rehydration, hyponatremia persisted because of excessive renal salt losses and concomitant enormous water losses, necessitating increasing amounts of sodium, up to 160 mmol/kg/d, and large amounts of intravenous fluids, up to 27 L/d. Highly increased levels of atrial natriuretic peptide (ANP) confirmed the diagnosis of CSW. The occurrence of a CSW has to be recognized early in the clinical course for adequate treatment and remains one of the important differential diagnosis of SIADH in hyponatremic states in patients with cerebral disorders, especially after head injury.