Of 11 patients with active acromegaly treated with transsphenoidal selective pituitary adenomectomy, four failed to show evidence of clinical improvement. An additional four patients showed clinical and biochemical improvement after the surgical procedure but, with more prolonged follow-up, showed a recurrence of acromegaly. Only three patients had prolonged clinical and biochemical evidence of improvement; they had glucose suppressed growth hormone concentrations of 1.0, 2.7, and 2.8 ng/mL 22, 40, and 24 months, respectively, after the surgical procedure. Two of these three patients, however, showed an increase in growth hormone concentration after thyrotropin-releasing hormone injection, a finding that suggests the possibility of eventual relapse. Our data also clearly indicated a need for more rigorous criteria for biochemical cure of acromegaly than those used previously. Study of 20 healthy volunteers showed that growth hormone concentration should normally be suppressible to less than 2.0 ng.mL after 100 g oral glucose.