This is the first report on a syndrome of inappropriate secretion of anti-diuretic hormone (SIADH) in a patient with recurrent ovarian cancer following carboplatin and paclitaxel administration. A 63-year-old woman received chemotherapy combining carboplatin and paclitaxel for recurrent ovarian serous papillary adenocarcinoma. Four days after the chemotherapy, she suffered decreased mental awareness and lost consciousness. Blood chemistry tests showed serum sodium of 109 mmol/l. Plasma osmolarity was reduced to 232 mOsm/kg while urine osmolarity was high at 430 mOsm/kg, strongly suggesting the presence of SIADH. Because hyponatremia was not observed in the subsequent cycle of chemotherapy consisting of weekly paclitaxel and cisplatin, carboplatin was thought to be responsible for the condition. Clinicians should be aware of the possibility that carboplatin may cause SIADH, and should carefully monitor electrolyte balance after chemotherapy.