Hypernatremia is defined by a serum sodium concentration of more than 145 mmol/L and reflects a disturbance of the regulation between water and sodium. The high incidence of hypernatremia in patients with severe brain injury is due various causes including poor thirst, diabetes insipidus, iatrogenic sodium administration, and primary hyperaldosteronism. Hypernatremia in the intensive care unit is independently associated with increased mortality and complications rates. Because of the rapid brain adaptation to extracellular hypertonicity, sustained hypernatremia exposes the patient to an exacerbation of brain edema during attempt to normalize natremia. Like serum glucose, serum sodium concentration must be tightly monitored in the intensive care unit.