Oral glucose-tolerance tests were done in 43 patients with acromegaly (38 with normal glucose tolerance and 5 with diabetes). Insulin levels were compared with 45 healthy London controls and levels found in a population survey in Western Australia from which a sixty-minute insulin-response index was derived. Although the mean serum-insulin levels were higher during the oral glucose-tolerance tests in non-diabetic acromegalics than in controls, only 10 out of 38 showed levels clearly outside the normal range. Only 9 were outside two standard deviations from the mean in the population survey. The insulin levels for the diabetics lay in the lower range for the rest of the group and they were less responsive to glucose. Apart from a possible correlation between mean serum-insulin and mean growth-hormone levels during the glucose-tolerance test no correlation was found between serum-insulin and indicators of disease activity. After treatment there was an appreciable fall in insulin levels only in those who initially had high insulin levels. The fall in serum-growth-hormone after treatment did not relate to the fall in insulin levels.