Thirty patients with active acromegaly were divided into five groups according to the results of their IRI and blood glucose curves in the course of oral glucose tolerance test. Young acromegalics with short duration of acromegaly had grossly exaggerated IRI response with the peak in the first half-an-hour and the flat blood glucose curve. In the second group the peak of IRI response was in the first hour and the blood glucose curve rose to high values in the early stage after the glucose load. The IRI response in the third group was still exaggerated but delayed and the diabetic type of blood glucose curve was found in some of these patients. Acromegalics with overt diabetes mellitus lost their exaggerated IRI response and the IRI curves were flat in those with more advanced disturbance of glucose tolerance. It was suggested that the groups described might represent individual stages in the development of diabetes mellitus in acromegaly. Unclear position in this scheme is kept by the group of active acromegalics with IRI response in normal limits and glucose tolerance unimpaired.