A case of hyponatremia following the first course of systemic adjuvant chemotherapy with cisplatin (CDDP) and 5-FU in a previously treated patient with esophageal cancer is reported. A 61-year-old man was admitted to our hospital for adjuvant chemotherapy after transthoracic esophagectomy and 3-field lymphadenectomy for esophageal cancer. Six days following chemotherapy, his serum sodium concentration was found to be 118 mEq/L, without edema or dehydration. This hyponatremic state was diagnosed as the syndrome of inappropriate secretion of antidiuretic hormone (SIADH) induced by CDDP, based on the hypo-osmolality of his serum and urine, and an inappropriate level of plasma vasopressin.