To investigate the fetoplacental leptin circulation in gestational diabetes, we compared cord leptin and insulin levels in 17 healthy pregnant women and 17 women with gestational-onset diabetes. Leptin levels in the umbilical arteries (mean+/-SD 1.80+/-0.76 ng/ml) were significantly (P<0.006) lower than those in umbilical veins (2.67+/-0.98 ng/ml) in normal pregnancies. Similarly, leptin levels in umbilical veins (mean+/-4.59+/-1.60 ng/ml) were significantly (P<0.001) higher than those in umbilical arteries (mean+/-SD 2.08+/-0.90 ng/ml) in gestational diabetes. However, leptin levels in umbilical veins were significantly higher (P<0.002) in gestational diabetes than those in controls. Additionally, in women with diabetes but not in controls, the birth weight and the cord leptin concentrations were positively related to cord insulin levels. We conclude that there is a hyperleptinaemia in the fetoplacental circulation in pregnant women with carbohydrate intolerance and in these cases insulin and leptin may have antagonist roles regarding fetal development.