In both diabetic and non-diabetic pregnancies fetal insulin is an important anabolic hormone. Fetal hyperinsulinaemia is associated with accelerated fetal growth and increased birth weight. Insulin and C-peptide concentrations in both umbilical cord and amniotic fluid reflect fetal beta-cell secretion and are correlated with birth weight. In the present study umbilical venous proinsulin and insulin concentrations were measured in 54 term infants born to women with and without mild disturbances of glucose tolerance. Umbilical venous cord proinsulin, assayed using a highly specific immunoradiometric assay, was independently correlated with infant birth weight (Rho = 0.53, p < 0.0001) and birth percentile (Rho = 0.65, p < 0.0001). The correlation between birth weight and birth percentile weight with umbilical venous insulin, measured using a non-specific insulin assay, was lost following correction for the influence of proinsulin. Umbilical venous cord proinsulin appears to be a good indicator of fetal beta-cell activity, and in this study, a superior marker to insulin assayed using a non-specific insulin radioimmunoassay. The longer half-life of proinsulin compared with insulin may contribute to proinsulin being a more robust marker of overall fetal beta-cell activity than insulin.