Rapid infancy growth predicts childhood obesity and earlier rate of maturation. We examined whether early growth rates might also influence levels of hormones relating to growth and weight gain by measuring IGF-I, IGF-II, and leptin levels in 497 normal 5-yr-old children who were followed closely from birth. IGF-I levels at 5 yr were unrelated to cord blood IGF-I levels at birth (r = 0.03; P = 0.7; n = 166) but were positively related to current weight (r = 0.32; P < 0.0005) and height (r = 0.30; P < 0.0005) and inversely related to birthweight (r = -0.21; P < 0.0005). By body composition, IGF-I levels correlated more closely with fat-free mass (r = 0.22; P < 0.0005) than with fat mass (r = 0.12; P < 0.05), whereas leptin (r = 0.57; P < 0.0005) and IGF-II levels (r = 0.15; P < 0.005) correlated more closely with fat mass. Independent of current body composition, IGF-I levels at 5 yr were significantly associated with rate of weight gain between 0-2 yr (beta = 0.19; P < 0.0005), and children who showed postnatal catch-up growth (i.e. those who showed gains in weight or length between 0-2 yr by >0.67 SD score) had higher IGF-I levels than other children (P = 0.02). IGF-II levels at 5 yr were positively related to IGF-II levels at birth (r = 0.17; P = 0.03; n = 166), and leptin levels at 5 yr were mainly related to current adiposity. Circulating IGF-I levels in childhood are influenced by infancy growth rates and possibly mediate the effects of early postnatal nutrition on later rates of growth and maturation.