A specific radioimmunoassay for insulin-like growth factor-binding protein 3 (IGFBP-3), developed against its binding subunit (IGFBP-3β), was used to investigate its diagnostic potential for the evaluation of growth disorders. Various factors were found to influence serum IGFBP-3. Most prominent was its considerable age dependence, showing low levels at birth and a peak at puberty. Its dependence on nutrition was obvious from a reduction by 16% after a 3-day fast. Low levels were observed in patients with impaired hepatic function due to biliary atresia, suggesting that the liver may be a major source. In end-stage renal failure, excessively high concentrations were found due to the accumulation of IGFBP-3-related low-molecular-weight forms, suggesting that the kidneys play an important role for clearance. The dependence of serum IGFBP-3 on GH was evident from a significant correlation with total spontaneous nocturnal GH secretion (n = 43; r = 0.62, p < 0.001). Constant levels over 24 h and a slow response to GH in patients with GH deficiency indicate slow kinetics of variation. Therefore, serum IGFBP-3 appears to reflect the integrated GH secretion over days. In patients with GH deficiency, IGFBP-3 levels were below the 5th percentile of the normal range (133 of 137). In contrast, children with normal-variant short stature (136 of 142) had levels above this limit. Normal IGFBP-3 levels were also found in Turner syndrome and Silver-Russell syndrome. IGFBP-3 was increased to the supranormal range in patients with pituitary gigantism (n = 2) and in acromegaly (12 of 13), whereas patients with familial tall stature had levels that clustered in the upper normal range. It is concluded (1) that serum IGFBP-3 is a highly informative screening parameter for GH deficiency, (2) that it is suited to detect GH excess and (3) that it may be used for therapy monitoring.