The question of if testing for growth hormone release is necessary in patients with chronic renal failure (CRF) is part of a greater debate. The question of what constitutes growth hormone deficiency (GHD) has become more controversial over the past few years. In some ways, the question has been replaced by the question, "Who will have a meaningful response to growth hormone (GH) therapy?" Since children with CRF generally respond to GH therapy, the question should be recast as, "When is testing for growth hormone release necessary in patients with CRF?" Why is the diagnosis of GHD important? A clear diagnosis of class GHD has many important implications for a patient. GHD is an easily treated cause of neonatal hypoglycemia. The diagnosis alerts the clinician to search for etiologies of GHD such as intracranial tumors and should stimulate a search for other pituitary deficiencies. Another important claim is that patients with classic GHD have a better long-term response to GH therapy. Children in other diagnostic categories, such as renal failure and Turner syndrome, also respond to GH therapy. Do diagnostic studies use to determine the function of the growth hormone-insulin-like growth factor (GH-IGF) axis help in the management of these children? Recently, experts have become increasingly interested in what constitutes a useful diagnostic test. To be a "good" diagnostic test, a procedure should have the following properties: (1.) have a rational connection to the disorder; (2.) good concordance with the diagnosis/outcome; (3.) accurate; and (4.) reproducible. Among tests that share these properties, the best test is generally the easiest and/or the least expensive. Many different tests can be used to evaluate the GH-IGF axis. These include GH stimulation tests, 24-hour GH profiles, IGF-I, and insulin-like growth factor binding protein 3 (IGFBP-3). High quality determinations of IGF-I and IGFBP-3 can be used to evaluate the GH-IGF axis.