The use of growth hormone (GH) for children with growth hormone deficiency (GHD) is well established. However, GHD is a syndrome that affects patients of all ages. Literature on pediatric GHD is extensive because treatment of this condition with recombinant GH replacement was approved about 12 years ago. Although GH-replacement therapy for adult GHD has been accepted practice in Europe for nearly 15 years, it was approved only recently for this indication in the United States. In adults, GHD has nonspecific symptoms, such as fatigue and impaired psychointellectual capacities, or no symptoms. Quantifiable alterations induced by GHD in adults may include altered body composition, reduced bone mineral density, impaired physical performance, abnormal lipid metabolism, and impaired quality of life. Appropriate patient selection for laboratory testing to confirm GHD is important because of the expense, difficulties, and risk inherent in its biochemical diagnosis. GHD is common in patients with treated or untreated pituitary tumors or other disorders of the pituitary, patients who have had cranial irradiation, and adults with a history of childhood-onset GHD. Isolated low levels of insulin-like growth factor-1 may indicate GHD but cannot substitute for an inadequate response of serum GH to insulin-induced hypo-glycemia or another GH secretagogue.