Serum levels of growth hormone were measured in 84 untreated acromegalic patients. The range of mean level during a glucose tolerance test was 8 to 1,860 ng./ml. and was lognormally distributed. Mean level was reproducible and correlated moderately well with maximum sellar area on lateral tomography. The hormone level was more responsive to oral glucose and insulin-induced hypoglycaemia in patients with smaller tumours. Attention is drawn to limitations of plain radiographs in assessing the size of a pituitary tumour. Suprasellar tumour extensions may be present in patients without visual field defects.