Previous studies have suggested that vascular reactivity may be altered in women with polycystic ovary syndrome (PCOS). We sought to evaluate the vascular reactivity specifically the endothelial function (EF) in women with PCOS and to study the effect of metformin on their EF.Thirty-one women diagnosed with PCOS and 33 healthy controls underwent evaluation of EF using a post-ischemia reactive hyperemia technique (Endo-PAT). EF was quantitatively determined as the ratio between the arterial pulse wave amplitude following a 5min arterial occlusion in the forearm to the pre-occlusion value. Oral metformin 850mg bid was administered to PCOS patients. After 3 months of treatment EF was reassessed using the same technique.The average endothelial function was 1.48±0.32 in the PCOS group versus 2.00±0.51 in the controls (P<0.001). There were no significant changes in the EF among the 18 patients who were treated with metformin, EF index pre- and post-treatment was 1.42 versus 1.43, respectively.Endothelial dysfunction is more likely to occur among PCOS patients than in healthy controls. These preliminary results suggest that metformin treatment for 3 months in PCOS patients does not improve endothelial function.