A 62-year-old woman was scheduled for an operation for ileus. Before the operation, we noticed severe hyponatremia (Na 117 mEq x l(-1)) probably due to dehydration. We corrected her hyponatremia slowly to avoid central pontine myelinolysis. Serum Na level increased to 131 mEq x l(-1) after surgery. She recovered from anesthesia without any neurologic problems.