The transurethral resection of the prostate (TURP) syndrome is caused by intravascular absorption of an electrolyte-free irrigating fluid during TURP for benign prostatic hypertrophy or prostatic carcinoma. The clinical symptoms and signs include hypertension, bradycardia, respiratory distress, hypotension, nausea, vomiting, confusion, blindness, seizure, coma, hyponatremina, and hypoosmolality. In this case, we incidentally detected very severe dilutional hyponatremia (99 mmol/L) without any symptoms during TURP and immediately took measures to treat TURP syndrome. On the third postoperative day, the hyponatremia had resolved within the normal range.