Objective：Delayed hyponatremia is a common complication after transsphenoidal surgery (TSS) for pituitary adenomas. We retrospectively evaluated our patients and tried to find possible associated risk factors of delayed hyponatremia. Methods：From 2006 until 2010, total 575 cases of pituitary adenomas were operated with TSS in a single institution. Thirty-one patients (5.3%) with delayed hyponatremia were compared to other 544 patients without delayed hyponatremia in the aspects of age, sex, Hardy classification, tumor size, hormonal type and activity on preoperative combined pituitary function test. Their postoperative outcomes were also evaluated in the aspects of tumor removal fashion and hormonal outcome on postoperative combined pituitary function test. Results：In 31 patients, delayed hyponatremia started 7.0 days after surgery (range : postoperative day 3-10). At the time of diagnosis of hyponatremia, serum level of sodium ranged from 111 mmol/L to 132 mmol/L (mean : 120.5 mmol/L) and mean duration of treatment was 4.l days (range : 2-9 days). Piecemeal tumor removal was more frequent in a group with delayed hyponatremia (p=0.035). The presence of postoperative hypopituitarism and aggravation of pituitary function were observed more frequently in a group with delayed hyponatremia (p=0.002 and p=0.017, respectively). Conclusion：Delayed hyponatremia is a common complication after TSS and potentially fatal. Piecemeal fashioned tumor removal, postoperative hypopituitarism and aggravation of pituitary function are associated with higher chance of delayed hyponatremia.