Growth hormone (GH) levels were measured after a 75g oral glucose load (OGTT) in normal adults, patients with impaired glucose tolerance (IGT), insulin-dependent diabetes mellitus (IDDM) and acromegaly. Nadir GH levels at 2-hour post-OGTT in normal subjects ranged from 0.4 to 8.4 mIU/L, the 95% confidence interval being 0.4-4.4 mIU/L. In IGT and IDDM subjects basal fasting GH levels were not significantly different from normal and did not alter during OGTT. The high fasting GH level measured in one each of the IGT and IDDM patients was suppressible at 1-hour after glucose intake. In contrast, acromegalic patients had elevated fasting GH levels (11.8-178 mIU/L) although in 3 patients, the levels were mildly elevated and overlapped with normal. OGTT failed or only partially suppressed GH secretion in all acromegalics. Therefore, elevated fasting GH levels are not diagnostic and OGTT is required for accurate diagnosis and assessment of treatment of acromegalic patients.