Disorders of sodium and water balance are frequent complications encountered in the care of critically ill neurologic patients. Common dysnatremias include hyponatremia caused by syndrome of inappropriate antidiuretic hormone (SIADH) and cerebral salt wasting syndrome (CSWS), as well as hypernatremia caused by central diabetes insipidus (CDI). A computed tomography (CT) scan of the head revealed a large amount of subarachnoid hemorrhage (SAH) with ventricular extension. As the major extracellular cation, sodium is one of the most important osmotic agents in the body. Hyponatremia is one of the most common electrolyte disorders in the neurologic patient and is frequently encountered in patients following SAH, traumatic brain injury, and many neurosurgical procedures. Treatment of hyponatremia depends upon patient symptomatology. Treatment of CDI depends upon both severity of symptoms and level of consciousness of the patient. Prompt recognition of hyponatremia and institution of appropriate, aggressive therapy is warranted to avoid complications.