A series of 44 patients with acromegaly underwent transsphenoidal surgery between 1987 and 1996. The early postoperative mean basal GH level <5ng/ml or <3ng/ml was achieved in 43 (97.7%) or 38 (86.4%) out of 44 patients, respectively. Preoperative abnormal GH secretory response to TRH, GnRH and oral glucose administration was restored to normal both after surgery and at the time of the final follow-up in all patients whose early postoperative mean basal GH levels were reduced to <3ng/ ml, whereas they remained abnormal in those with mean basal GH levels of ≥3ng/ml. In contrast, insulin-like growth factor 1 (IGF-1) levels, when measured by the extraction method, tended to be reduced gradually to normal between 6 months and 2 years after surgery in some patients with a successful operation. Therefore, 34 (87.1%) out of 39 patients who have been followed up longer than 6 months met the following stringent criteria at the time of the final follow-up: mean basal GH level <3 ng/ml, a normal IGF-1 level, and normal GH response to TRH, GnRH and oral glucose administration. In this series, the most unfavorable preoperative factor influencing operative outcome is tumor invasion of the cavernous sinus. Our results clearly indicate that selective adenomectomy by transsphenoidal surgery is the therapy of first choice in any patient with acromegaly and that the complete biochemical cure of acromegaly can be achieved in 87% of patients by surgery alone with an acceptable low surgical morbidity.