Experience in the management of 100 cases of acromegaly is described. Three quarters of these had been referred directly to the endocrine clinic at the Middlesex Hospital. The remainder were referred from the Royal Post-graduate Hospital because they were thought unsuitable for yttrium implantation. The patients were studied by clinical assessment of severity, by measurement of basal growth hormone levels on three separate mornings, and by a review of possible complications. Particular attention was paid to diabetes, hypertension, cardiomegaly, respiratory, vascular and skeletal changes as well as visual field defect. Aggressive treatment was recommended in 77 patients. It was not recommended in the remainder on account of age, intercurrent illness or the apparent mildness of the condition. Fifty-nine patients were treated by trans-sphenoidal hypophysectomy. In 46 of the 59 patients the mean basal growth hormone level has been reduced to 5 ng/ml or less. In 39 this followed operation, in five operation and subsequent X-ray therapy and in two operation and the continuing effect of previously implanted yttrium. Of these 46 patients in whom the growth hormone level has been reduced to normal, 26 do not show any deficiency of anterior pituitary trophic hormones, 13 have gonadotrophin defect (in eight of these it was present before the operation) and seven require full replacement therapy. One patient died at home six weeks after the operation from a pulmonary embolus. There was one case of CSF rhinorrhoea which stopped spontaneously and three of acute frontal sinusitis. Trans-sphenoidal hypophysectomy is shown to be an effective means of treating acromegaly. If the basal level of growth hormone is not reduced to normal by six weeks after operation, it is recommended that a course of X-ray therapy should be given. This does not apply if irradiation has been used before operation.