To investigate the role of growth hormone (GH) and its downstream axis in normal growth and growth disorders, we measured serum GH-binding protein (GHBP) levels in children by ligand-mediated immunofunctional assay (LIFA). Samples were taken from 512 healthy children of normal stature, 146 healthy neonates, 153 short but otherwise normal children, and 27 patients with GH deficiency. Serum GHBP showed no significant diurnal variation. Serum GHBP levels were low in neonates, acutely rose within 6 months, and gradually increased toward midpuberty in normal children (r = 0.2, P < 0.01, assessed among subjects over 1 year old). Neither significant sexual dimorphism nor rapid pubertal changes were observed. In normal short children and patients with GH deficiency, GHBP was lower than normal, but not significant. These data suggest that one of the etiologies for growth disorder in children could be resistance to, or low efficacy for, utilization of GH.