OBJECTIVE--To test the hypothesis that reduced fetal growth leads to altered plasma insulin-like growth factor-1 (IGF-1) concentrations in childhood. DESIGN--A follow up study of 4 year old children whose birth weights were recorded, and of 7 year old children whose weight, length, head circumference, and placental weight were measured at birth. SETTING--Pune, India, and Salisbury, England. SUBJECTS--200 children born during October 1987 to April 1989 in the King Edward Memorial Hospital, Pune, weighing over 2.0 kg at birth and not requiring special care, and 244 children born during July 1984 to February 1985 in the Salisbury Health District and still living there. MAIN OUTCOME MEASURE--Plasma IGF-1 concentrations. RESULTS--In both groups of children, and consistent with findings in other studies, plasma IGF-1 concentrations were higher in taller and heavier children, and higher in girls than boys. Allowing for sex and current size, concentrations were inversely related to birth weight (Pune p = 0.002; Salisbury p = 0.003). Thus at any level of weight or height, children of lower birth weight had higher IGF-1 concentrations. The highest concentrations were in children who were below average birth weight and above average weight or height when studied. Systolic blood pressures were higher in children with higher IGF-1 concentrations (Pune p = 0.01; Salisbury p = 0.04). CONCLUSIONS--Children of lower birth weight develop higher circulating concentrations of IGF-1 than expected for their height and weight. This is consistent with the hypothesis that under-nutrition in utero leads to reprogramming of the IGF-1 axis. The increase of plasma IGF-1 concentrations in low birthweight children may also be linked to postnatal catch-up growth. High IGF-1 concentrations may be one of the mechanisms linking reduced fetal growth and high blood pressure in later life.