In this study 19 patients were preoperatively treated with octreotide for 1-17 months (mean, 5 months), with doses from 150-1500 microg daily, and those patients were matched to 19 untreated patients with comparable tumor classification and preoperative serum GH concentrations. Octreotide was started at 300 microg daily by s.c. injections or continuous sc infusion using a pump in increasing doses, depending on the responses of the serum GH and insulin-like growth factor I (IGF-I) concentrations. During pretreatment, seven patients achieved a serum GH concentration below 5 mU/L, whereas six patients normalized their serum IGF-I. Postoperatively, a serum GH concentration below 5 mU/L was achieved in 15 pretreated and 14 untreated patients, a normal serum IGF-I level (<2 SD) was achieved in 10 pretreated and 15 untreated patients, and normal serum GH suppression during GTT was reached in 12 treated and 14 control patients. No differences were found in complication rate or incidence of hypopituitarism caused by surgery. Adjuvant therapy was required in 7 treated and 5 untreated patients. At follow-up examination, 5.7 and 4 yr postoperatively, 10 pretreated and 12 control patients could be considered cured by surgery only, according to our criteria for remission (serum GH, <5 mU/L; normal GH suppression and normal serum IGF-I). In summary, we found no difference in direct postoperative and follow-up results of transsphenoidal surgery between pretreated patients and untreated patients. This finding is in discordance with other studies, which have claimed a beneficial effect of octreotide pretreatment.