The normal values of insulin-like growth factor I (IGF-I), IGF-binding proteins 1 and 3 (IGFBP-1 and IGFBP-3), and the high-affinity growth hormone binding protein (GHBP) are not well established in large series of healthy fullterm newborns. We report the normative data for IGF-I, IGFBP-I, IGFBP-3, and GHBP in 271 normal Spanish full-term newborns, born between 37 and 42 weeks of gestation, and compare these results with the same parameters studied in 39 premature infants. Furthermore, we report the relationship between results found in the normal full-term newborns and those of 252 healthy prepubertal (Tanner stage I) Spanish children. Serum GHBP, IGF-I, and IGFBP-3 levels are very low in the premature infant and show a significant increase in full-term newborns, and continue to decline during childhood (p < 0.001; analysis of variance). A positive correlation between GHBP, IGF-I, and IGFBP-3 versus gestational age was observed. In contrast, we found a negative correlation between IGFBP-I and gestational age. There is a direct relationship between the ponderal index and IGF-I and IGFBP-3. When the group of premature newborns was divided into infants born before or after 32 weeks of gestation, we found higher levels of IGF-I and IGFBP-3 (p < 0.01 and p < 0.05, respectively, by Student's test) in the group with the higher gestational age; however, the IGFBP-I level was lower in this group (p < 0.001 by Student's t test), with no differences seen in serum GHBP concentrations. The presence of IGFBPs in the premature infant suggests that they are important modulators of IGF-I action during fetal growth and development.