The systemic absorption of the flush liquid, including sorbitol, glycine or mannitol, can lead to complications, such as hyponatremia, volume overload and pulmonary or cerebral edema. Acute hyponatremia is defined as a reduction in the plasma sodium level in less than 48 h. Acute symptomatic hyponatremia should be corrected aggressively because it may cause irreversible neurological damage and death. Rapid correction of hyponatremia causes severe neurologic deficits, such as central pontine myelinolysis; thus, the optimal therapeutic approach has been debated. This article examined acute symptomatic hyponatremia in a patient undergoing transcervical myomectomy for a submucosal myoma. A thirty-seven-year-old patient was evaluated in obstetrics and gynecology clinic because of altered mental status and agitation. There was no history of chronic illness or drug use. It was discovered that during the operation, 12 L of the flush fluid, which contained 5 % mannitol, had been infused, but only 7 L of the flush fluid had been collected. On physical examination, the patient's general condition was moderate, her cooperation was limited, she was agitated, and her blood pressure was 120/70 mmHg. The sodium level was 99 mEq/L. Furosemid and 3 % NaCl solution were given. Her serum sodium returned to normal by increasing 39 mEq/L within 14 h. Her recovery was uneventful, and she was discharged 24 h after her serum sodium returned to normal. In conclusion, if there is a difference between the infused and collected volumes of the mannitol irrigant, severe hyponatremia may develop due to the flush fluid used during transcervical hysteroscopy and myomectomy. In these patients, acute symptomatic hyponatremia may be corrected as rapidly as the sodium level dropped.