SUMMARY Assays for human growth hormone (HGH) were carried out on 89 acromegalic patients, 81 of whom were studied before any treatment had been given. Serial studies were undertaken, generally at 6-monthly intervals, with the same test procedure, using a 50 g oral glucose tolerance test (GTT) and identical assay conditions over a period of 8 years. Twenty-three patients were assessed at intervals during periods of up to 4 years whilst they remained untreated. The general picture was one of unchanging HGH levels. Ten patients were studied before and after external irradiation. HGH levels showed a slow continuing fall for as long as 4 years and thereafter they stabilized at one-third of pretreatment values. HGH levels in 12 patients treated with radioactive implants showed an immediate fall to one-third, and thereafter a further slow decline to one-tenth of pretreatment levels. The response in eight patients treated by surgical removal of pituitary tissue and subsequent radiotherapy varied considerably. No patient, treated or untreated, showed evidence of partial suppression of HGH secretion during the GTT although three patients consistently responded to glucose with paradoxical hypersecretion.