ObjectiveTo investigate the changes of leptin levels in serum, cord venous blood and placenta and examine the possible relationships among them in women with gestational diabetes mellitus (GDM).Methods This case-control study was performed in 40 women with GDM and 40 normal women. The women with GDM received dietary advice, blood glucose monitoring and insulin treatment when necessary. Matal serum and venous cord blood leptin, insulin and C-peptide levels were detected by ELISA. The expression level of leptin in placenta was measured by an immunohistochemical method. Results1. Leptin, insulin and C-peptide levels in serum of the women with GDM were significantly higher than those of the normal women (P<0.01,P<0.05, P<0.01). 2. Cord venous blood leptin and insulin in GDM group were significantly higher than in the normal group (P<0.01,P<0.05). No significant difference was found in cord venous blood C-peptide between two groups. 3. The expression of placenta leptin protein was significantly higher in GDM women than in normal women (P<0.01). There were positive correlations between placental leptin, cord venous leptin and insulin, birth weight, ponderal index in GDM women (r=0.37,P<0.05;r=0.39,P<0.05;r=0.53,P<0.01;r=0.54,P<0.01). However, there was no correlation between placental leptin and serum leptin.Conclusion The women with GDM and their fetuses suffer from hyperleptinaemia and hyperinsulinaemia. In the women with GDM, there was disorder in the interaction between leptin and insulin. The GDM women and their fetuses may be prone to insulin resistance and leptin resistance in late pregnancy.