Patients with intracranial disorders are prone to develop hyponatremia with inability to prevent the loss of sodium in their urine. This was originally referred to as "cerebral salt wasting," but more recently is thought to be secondary to the syndrome of inappropriate secretion of antidiuretic hormone (SIADH). Blood volume determinations were made in 12 unselected neurosurgical patients with intracranial disease who fulfilled the laboratory criteria for SIADH. Ten of the 12 patients had significant decreases in their red blood cell mass, plasma volume, and total blood volume. The finding of a decreased blood volume in patients who fulfill the laboratory criteria for SIADH is better explained by the original concepts of cerebral salt wasting than by SIADH. The primary defect may be the inability of the kidney to conserve sodium.