The glucose and immunoreactive insulin (IRI) responses to a 100 g oral glucose tolerance test (oGTT) administered to 23 acromegalic patients before and after transsphenoidal adenomectomy and to a sex and age-matched control group were studied. The preoperative growth hormone levels, ranging from 11 to 360 mU/l, had normalized (i.e. less than or equal to 5 mU/l) after surgery in all cases. Before surgery paradoxical increases in the GH level, either after i.v. TRH injection or during the oGTT were seen in 14 patients. After surgery, the paradoxical reaction to the oGTT had normalized in all cases and the increase measured after the TRH injection normalized in nine out of 12 cases. The disturbed glucose tolerance (either impaired tolerance or frank diabetes) was cured by surgery. The plasma glucose levels determined 1 and 2 years after surgery did not differ from those found for the control group. Preoperative plasma IRI levels were significantly elevated, whereas after surgery the IRI levels had decreased. Fasting levels were normalized in all patients. Two years after surgery, eight patients still showed some abnormalities of the insulin secretion (as revealed by the integrated sum and the maximum increment in IRI levels during the oGTT) despite the presence of normal circulating GH levels and a normalized paradoxical reaction to TRH. The data show that after normalization of GH levels in acromegalics, increased insulin secretion may still occur after glucose ingestion in some patients. It is suggested that this abnormality could be the result of an increased pancreatic islet beta-cell mass, as a persisting abnormality of the acromegalic state, although peripheral insulin resistance cannot be excluded totally.