Objectives. —To investigate the occurrence of hyponatremic encephalopathy and the effects of therapy for symptomatic hyponatremia on outcome in women undergoing endometrial ablation. Design and Setting. —Consultations from university-affiliated and community hospitals from June 1991 to June 1992. Patients. —Four generally healthy women who underwent elective endometrial ablation for dysfunctional bleeding and developed hyponatremic encephalopathy. Interventions. —Three patients were promptly treated with hypertonic (514 mmol/L) sodium chloride such that the serum sodium level increased from 102 to 123 mmol/L within 24 hours. The fourth patient was not treated until after suffering grand mal seizures followed by respiratory arrest. Main Outcome Measures. —Of the four patients who developed hyponatremic encephalopathy, the diagnosis was established before respiratory arrest occurred in three. The operative procedure was terminated and all three were aggressively treated with intravenous hypertonic sodium chloride, which raised the serum sodium level to modestly hyponatremic levels (120 to 130 mmol/L). All three completely recovered without sequelae. The fourth patient suffered respiratory arrest before therapy could be initiated. The patient remained comatose, and central diabetes mellitus and insipidus developed. She never regained consciousness and died after several days. Autopsy revealed cerebral edema and tonsillar herniation. Results. —The mean (±SD) preoperative serum sodium level was 138±1 mmol/L; at the time of diagnosis of hyponatremia, it was 107±13 mmol/L. In two patients, hyponatremic encephalopathy was diagnosed intraoperatively because of tremulousness and either hypothermia or hypoxemia. In the other two patients, the diagnosis was made postoperatively because of headache, nausea, emesis, and in one of these patients, respiratory arrest. Conclusions. —Women undergoing elective endometrial ablation can develop severe symptomatic hyponatremia, which can be fatal. The presence of symptoms suggesting hypo-osmolality should lead to immediate measurement of plasma sodium level. If hyponatremia with hypo-osmolality is present, early and appropriate therapy for the hyponatremia should be instituted before respiratory insufficiency occurs. (JAMA. 1993;270:1230-1232)