Objective: To investigate the protein and messenger RNA expression of glucose transporter 4 in endometrium of women with polycystic ovary syndrome (PCOS), and to evaluate its change after three months treatment of metformin.Methods.Twenty-two patients with polycystic ovary syndrome (group A) and six non-PCOS infertile women (group B) were recruited in our hospital.The consent form was obtained from each patient.Endometrium and blood samples were obtained during the proliferative phase of the menstrual cycle.Reverse transeriptase-polymerase chain reaction (RT-PCR) and immunohistochemical method were applied to detect the expression of glucose transporter 4 (GLUT4) in endometrium.All PCOS patients received monotherapy of metformin after endometrium biopsy.Seven un-conceived patients (group A1) from group A who completed three months of metformin treatment were selected to perform the second time biopsy during proliferative phase.The expression of GLUT4 were remeasured as well.Results.There were no significant differences of the levels of E2, P and endometrium thickness on the biopsy day between group A and group B.But the basal levels of LH, T, LH/FSH ratio, and the ovarian volume were significantly higher in group A as compared with group B (P＜0.001).The expression of GLUT4 in group A was significantly lower than that of group B (1.05±-0.13 vs 1.50±0.21, P＜0.001).In group A1,the expression of GLUT4 in endometrium were changed from 1.08±0.08 to 1.27±0.16 before and after treatment (P＜0.05).The results of immunohistochemical staining of GLUT4 in endometrium were coincident with the results of RT-PCR.The fast insulin level and the insulin sensitivity index were also improved obviously after three months of metformin therapy (P＜0.05).Conclusions: Insulin-resistance status was proved existing in endometrium of PCOS women; A significant improvement of GLUT4 expression was observed in endometrium after metformin treatment, supporting that metformin can relief insulin-resistant status of the endometrium in PCOS patients.