We present a case of reversible encephalopathy syndrome caused by hyponatremia during neoadjuvant chemotherapy with 5-FU+CDDP for esophageal cancer. A 52-year-old woman visited another hospital with the complaint of discomfort during swallowing. Upper gastrointestinal endoscopy revealed an ulcerated lesion in the lower thoracic esophagus, which was pathologically diagnosed as squamous cell carcinoma. She was admitted to our institution, and the diagnosis of esophageal cancer Mt-Lt, type 2, cT2N1M0, stage II was made. We obtained informed consent for her treatment with both CRT and surgery, and she selected surgery. Therefore, she was administered neoadjuvant chemotherapy with 5-FU+CDDP therapy. On day 4, abnormalities of the central nerve, such as unconsciousness, difficulty in speaking, and muscle cramps were, observed. After the examinations she was diagnosed with encephalopathy. It should be noted that hyponatremia induced by renal sodium wasting syndrome(RSWS)was considerd to be the cause, and we report our case with the literature cited.