A 66-year-old woman with small-cell lung cancer was administered chemo-radiotherapy consisting of cisplatin (CDDP) and etoposide (ETP). From day 3, she developed vomiting and hyponatremia that persisted despite fluid infusion and cortico-steroid administration. On day 7, the hyponatremia worsened (serum sodium level, 109 mEq/L), leading to disturbed consciousness and convulsions. The serum sodium level gradually increased after intravenous administration of hypertonic saline; on day 22, the serum sodium level was almost normal without any neurological implication. We diagnosed this clinical condition as renal salt-wasting syndrome (RSWS) on the basis of dehydration and high urinary sodium excretion at the onset. In the second course of chemotherapy, CDDP was replaced with carboplatin (CBDCA); consequently, hyponatremia was not observed. Hyponatremia that develops after the administration of CDDP may be due to not only the syndrome of inappropriate secretion of anti diuretic hormone (SIADH) but also RSWS. When RSWS is suspected, hypertonic saline should be administered.