The biochemical diagnosis of acromegaly and assessment of the clinical activity of the disease can be made by measurement of the insulin-like growth factor-I (IGF-I) concentration in the serum. False-positive increases seldom occur. This determination has largely replaced measurement of the growth hormone (GH) response to the oral administration of glucose, which in normal individuals suppresses to less than 2 μg/L. In normally fed individuals with normal liver and renal function, circulating IGF-I levels reflect the integrated effect of GH at the tissue level and also correlate with mean 24-hour GH levels. Measurement of the IGF-I concentration differentiates subjects with active untreated acromegaly from normal individuals, but the serum IGF-I level also reflects the clinical activity of the disease. However, it remains debatable whether this particular parameter represents most faithfully the course of the disease. Computed tomography and magnetic resonance imaging have greatly facilitated the diagnosis of pituitary abnormalities in acromegalic patients. Evidence of a pituitary tumor is found in almost all patients. Most adenomas are larger than 10 mm in diameter at the time of diagnosis, and extrasellar extension is present in approximately one third of cases.