Serum growth hormone levels, thyroid function, and adrenal function were measured before and after surgery in 16 of 17 acromegalic patients undergoing undergoing transnasal transsphenoidal microsurgery of the pituitary. Thirteen patients have been followed up for 12 to 24 months; three patients have been followed up for three to six months. Serum growth hormone levels decreased to less than 5 ng/ml in seven of nine previously untreated patients; thyroid and adrenal function were preserved in eight of these nine patients. In seven patients treated previously by other modes of therapy, growth hormone levels after transsphenoidal surgery decreased to less than 5 ng/ml in three, to between 5 and 10 ng/ml in three, and from 98 to 41 ng/ml in one. Preoperative adrenal function was normal in six of these seven patients and was preserved in four; thyroid function was normal in five patients preoperatively and was preserved in three. Transsphenoidal microsurgery appears to offer an effective means of lowering growth hormone levels and a possiblity of preserving any remaining normal pituitary function. It may be considered for initial treatment in selected patients in whom more rapid arrest of acromegaly is indicated.