The concentration of circulating insulin-like growth factor I (IGF-I) is a potential marker for growth hormone (GH) deficiency in adults. Indeed, researchers have shown that IGF-I levels are of greater diagnostic value than other possible markers, such as IGF-binding protein 3 (IGFBP-3) and the acid-labile subunit (ALS). Accurate age-matched normative data are essential to give patient data diagnostic meaning. Such data are assay specific and must exclude those individuals with certain confounding medical conditions. Post-diagnosis, monitoring of IGF-I and IGFBP-3 levels can be used to assess the efficacy and safety of GH replacement therapy. Furthermore, IGF-I levels, and possibly ALS levels, can be used to aid the diagnosis and monitoring of acromegaly. For example, acromegaly can be excluded in patients with normal IGF-I levels if liver failure and malnutrition/malabsorption are ruled out.