Background:We analyzed the role that nutrition and the insulin-like growth factors IGF-I and IGFBP-3 play on neonatal growth.Methods:Full-term and preterm infants with 1 and 3 weeks of postnatal life (n = 54 and n = 33, respectively) were studied. Anthropmetric variables, daily intake of energy and nutrients, and serum levels of IGF-I and IGFBP-3 were measured.Results:At the first week after birth, preterm infants had lower IGF-I levels than did those in the control group. At the third week of postnatal life, serum IGF-I and IGFBP-3 levels showed a significant increase. Preterm infants born before 33 gestational weeks showed lower IGF-I (p < 0.02) and IGFBP-3 (p < 0.02) levels than those born between 33 and 37 gestational weeks. Preterm infants fed with human milk supplemented with a formula showed higher serum IGF-I levels than those fed exclusively with a milk formula (mean ± SEM 48.2 ± 9.5 μg/L vs. 25.4 ± 4.4 μg/L, p < 0.05). IGF-I and IGFBP-3 were correlated between themselves and with energy and protein intake. Multiple regression analysis confirmed that energy intake and serum IGFBP-3 levels were the most predictable variables with regard to IGF-I levels at neonatal period.Conclusions:These feedings suggest that IGF-I levels during the neonatal periods are influenced by the maturity stage of the newborn, energy intake, and the type of lactation.