Fifty-nine acromegalic patients, transsphenoidally operated by a single neurosurgeon (H.v.D.) were followed for at least 10 yr to assess the late outcome of surgery. Mean follow-up was 16 +/- 0.4 yr (range, 10-22). Criteria for remission were a serum GH concentration below 2.5 microg/L, a normal glucose-suppressed GH (oral glucose tolerance test), and a normal serum insulin-like growth factor I (IGF-I) concentration. Mean serum GH concentration decreased from 59 +/- 8.7 microg/L to 5.6 +/- 1.4 microg/L after surgery. Early postoperative remission rates were 61% (GH, <2.5 microg/L), 67% (suppressed GH), and 60% (both GH <2.5 microg/L and suppressed GH). Early postoperative remission was significantly related to preoperative serum GH concentration (P: = 0.023), but not to tumor size. Of 36 patients with postoperative remission (GH, <2.5 microg/L), 9 patients received (prophylactic) radiotherapy for persistent paradoxical reaction to TRH or probable invasive tumor growth. All nine patients are in remission at the end of follow-up. Of the other 27 patients with postoperative remission, 5 (19%) developed recurrence, becoming evident within 5 yr in 4 patients and after 10 yr in 1 patient. Of these 27 patients, surgical remission rates at the end of follow-up are 78% (random GH, <2.5 microg/L), 73% (normal glucose-suppressed GH), 74% (normal IGF-I), and 65% (normal IGF-I and GH suppression). Of the patients with postoperative persistent disease, 18 patients were irradiated and 5 patients were followed without further treatment. Two of five nontreated patients had spontaneous normalization of GH concentration at the 6 months visit and remained in remission by surgery only. The long-term efficacy of multimodality treatment was evaluated after exclusion of the prophylactically irradiated patients. At the end of follow-up, 48% of patients had not required adjuvant therapy and the rest received radiotherapy (34%), octreotide (10%), or both (8%). Remission rates of multimodality therapy were 96% (serum GH, <2.5 microg/L) and 94% (normal serum IGF-I concentration). Remission rates of transsphenoidal surgery alone were 46% (serum GH, <2.5 microg/L), 44% (normal IGF-I concentration), 41% (suppressed serum GH), and 37% (normal serum IGF-I and suppressed GH). In this first report on separate 10 or more years results of transsphenoidal surgery for acromegaly, using strict criteria for remission, 19% of patients with postoperative remission developed recurrence. Nevertheless, about 40% of patients remain in remission after only surgical intervention, even after a mean follow-up of 16 yr.