The incidence and risk factors of symptomatic and asymptomatic hyponatremia were investigated in 94 patients who underwent transsphenoidal surgery and serum sodium level monitoring between January 2002 and December 2006. The records were retrospectively reviewed to determine the incidence and risk factors (age and sex, tumor size, endocrinologic findings) of hyponatremia. Postoperatively, the serum sodium levels of the patients were measured at least once within 2 or 3 days. Hyponatremia was found in 17 of the 94 patients, of whom 7 became symptomatic. The mean sodium level of symptomatic patients with hyponatremia at diagnosis was 123.5 mEq/l, compared with 129.8 mEq/l of asymptomatic patients. The serum sodium levels began to fall on mean postoperative day 7 and reached nadir on mean day 8. All 17 patients with hyponatremia were treated with mild fluid restriction. Four symptomatic patients with severe hyponatremia were treated with 3% hypertonic saline infusion in addition to fluid restriction. One symptomatic patient with severe hyponatremia was treated with fluid restriction only. All patients recovered within 5 days of management. Sex, tumor type, and tumor size did not correlate with development of delayed hyponatremia, but patients aged ≥50 years were more likely to develop hyponatremia. Postoperative hyponatremia after transsphenoidal surgery is more common than previously reported and may lead to fatal complications. Therefore, all patients should undergo serum electrolyte level monitoring regularly for at least 1 or 2 weeks after transsphenoidal surgery.