Background: To investigate and differentiate the causes of hyponatremia in an 8-y old boy. Methods: An 8-y boy presented with headache, vomiting, and diplopia. Magnetic resonance imaging of the brain confirmed a mass in the pineal region. Pathology report demonstrated a mixed germ cell tumor with a yolk sac component. A multi-agent chemotherapy and radiation regimen was initiated. He developed hyponatremia, with sodium concentrations varying from 116 to 133. mEq/l. Results: Serum levels of sodium, chloride, phosphorous, uric acid, and osmolality were low. Serum α-fetoprotein, β-HCG, and lactate dehydrogenase were highly elevated. Urine sodium and osmolality were increased. Conclusions: These presentations suggest that the patient has cerebral salt-wasting syndrome caused by intracranial germ cell tumor. Recognition and differentiation of cerebral salt-wasting syndrome from other disorders are essential.