Small preterm infants experience a unique postnatal period characterized by slow growth, inadequate nutrition and growth inhibiting treatments. Many have already been growth-restricted in utero. Studying this period is important when developing growth optimizing strategies for these infants and, in a broader context, as a model of extreme conditions that restrict growth. By following short-term growth of 48 very-low-birth-weight (VLBW; birth weight <1,500 g) infants for 9 postnatal weeks, we found that circulating insulin-like growth factor (IGF)-I and IGF binding protein (IGFBP)-3 levels are low and reflect rigorously measured (knemometry and weight) concurrent growth velocity. Moreover, weight growth velocity is correlated with the ratio of lesser to highly phosphorylated IGFBP-1 but not with absolute IGFBP-1 concentrations. Thus, IGF-I, IGFBP-3 and the phosphorylation status of IGFBP-1 in circulation are likely to be involved in growth regulation during the postnatal period in VLBW infants.