A 41-year-old female complaining of easy fatigue, headache, thickened extremities and deepened facial folds was admitted to the hospital. As early as 6 years ago, she had been told by family members that her voice was changed and both hands was thickened. Cranial plain radiography indicated ballooning of sella turcica. Sella MRI disclosed 2.2cm sized pituitary tumor with suprasellar extension. The pituitary tumor with hypothalamic extension was removed via a transsphenoidal approach. The postoperative course was uneventful until hyponatremia with sudden headache and respiratory arrest was developed. The case was due to hyponatremic encephalopathy on the basis of the clinical course and symptoms, and 3% hypertonic saline was infused for 12 hours until the hyponatremia was corrected. We have experienced a mortality case of hyponatremic encephalopathy in which surgieal removal of a pituitary tumor from a female acromegalic patient in menstruant period was followed by the sudden occurrence of hyponatremia, which in turn was later associated with marked brain edema.