Hyponatremia is the most frequent electrolyte disorder in critically neurological patients. Cerebral salt wasting syndrome (CSW) is defined as a renal loss of sodium during intracranial disease leading to hyponatremia and a decrease in extracellular fluid volume. The pathogenesis of this disorder is still not completely understood. Sympathetic responses as well as some natriuretic factors play a role in this syndrome. Distinction between SIADH and CSW might be difficult. The essential point is the volemic state. It is necessary to rule out other intermediate causes. Treatment requires volume replacement and maintenance of a positive salt balance. Mineral corticoids may be useful in complicated cases.