Transsphenoidal resection of adrenocorticotrophic hormone (ACTH)-producing pituitary adenomas has the potential of curing most patients with Cushing's disease. However, transsphenoidal exploration of the pituitary is not always curative, and patients who have remission of hypercortisolism following surgery occasionally develop a recurrence. Whether repeat pituitary surgery should be performed for recurrent or persistent Cushing's disease has not been evaluated previously. To determine the efficacy of transsphenoidal surgery in recurrent or persistent Cushing's disease, we performed transsphenoidal surgery in 31 patients (22 women and nine men) who had previously undergone a transsphenoidal operation and two female patients who had had previous pituitary irradiation only. In 24 (73%) of the 33 patients, remission of hypercortisolism was achieved by surgery. Although preoperative computerized tomography (CT) scanning identified an adenoma in only three of the 33 patients, in 20 patients a discrete adenoma was identified at pituitary exploration. The incidence of hypercortisolism was greatest if an adenoma was identified at surgery and the patient received selective adenomectomy (19, or 95% of 20 patients), if there was evidence at surgery or by preoperative CT scanning that the previous surgical exposure of the pituitary was incomplete (seven, or 78% of nine patients), if an adenoma was seen on preoperative CT scanning (three of three patients), or if the patient had had prior pituitary irradiation without surgery (two of two patients). In contrast, only five (42%) of 12 patients who received subtotal or total hypophysectomy had remission of hypercortisolim. Surgically induced hypopituitarism occurred in six (50%) of these 12 patients, but in only one (5%) of the 20 patients who underwent selective adenomectomy. Three (13%) of the 24 patients who were in remission from hypercortisolims following repeat surgery developed recurrent hypercortisolism 10 to 47 months postoperatively. Repeat transsphenoidal exploration of the pituitary and treatment limited to selective adenomectomy should be considered in patients with hypercortisolism despite previous pituitary treatment. If an adenoma is identified during surgery, the chance of remission of Cushing's disease is high and the risk of hypopituitarism is low; however, if no adenoma can be found and partial or complete hypophysectomy is performed, remission of hypercortisolism is less likely and the risk of hypopituitarism is about 50%.