Hyponatremia is a frequent event in neurosurgery practice and is usually associated with subarachnoid hemorrhage, head trauma, infections and neoplasms. The two common clinical manifestations are the inappropriate secretion of antidiuretic hormone (SIADH) and the cerebral salt wasting syndrome (CSWS), which were usually attributed to each other due to identical clinical presentation. In contrast to the better-recognized SIADH, there has not been a uniform consensus over the humoral and neural mechanisms of CSWS and functional aspects of renal response. in this article, we report on 2 cases of a primitive neuroectodermal tumor with prolonged CSWS manifested during the intraventricular dissemination of primary disease and the high catabolic stage. Copyright (C) 2000 S.Kaiger AG,Basel.