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To highlight the use of insulin-like growth factor-I (IGF-I) measurements in the management of acromegaly.The physiologic and pathophysiologic regulation of growth hormone (GH) and IGF-I secretion and the role of measurements of these two variables for diagnosing and monitoring the acromegalic condition are discussed.Chronic GH hypersecretion by pituitary tumors leads to the clinical condition of acromegaly, characterized by enlargement of membranous bones and soft tissue overgrowth. Because of the frequent delay in diagnosis of acromegaly, disease management often involves the use of multiple therapeutic modalities. Therefore, adequate patient management necessitates precise diagnostic tests to (1) establish that the acromegalic condition exists, (2) determine the degree of severity, and (3) assess the effectiveness of treatment in patients who have received therapy. Because the predicted mortality rate among patients who have achieved a biochemical remission is substantially improved in comparison with those who have biochemical indices that indicate residual disease activity after therapy, a biochemical test is required to determine the necessity of undertaking further treatment. The GH-dependent peptide IGF-I provides useful information for making this decision. In addition to providing data about the degree of GH hypersecretion, IGF-I concentrations reflect the severity of disease at the time of initial assessment and the degree of improvement in response to any therapeutic intervention. Direct measurement of GH after glucose suppression is the best indicator of residual tumor secretory activity. IGF-I measurements should be performed in a reference laboratory with adequate age-adjusted normative ranges and a high degree of precision, which allows serial determinations over time to reflect the degree of biochemical improvement. Measurement of IGF-I often correlates with the degree of persistent GH hypersecretory abnormality after therapy, although no precise correlation is found in some cases.Measurements of GH suppression after glucose administration and of IGF-I after treatment of patients with acromegaly provide useful complementary information. Every effort should be made to achieve normalization of these biochemical indices so that patients are offered the best therapeutic prognosis.