Polycystic ovary syndrome (PCOS) is a heterogeneous disorder, which is considered not only a reproductive disease but also a metabolic disorder associated with long-term health risks. The aim of this study was to assess the effects of metformin on insulin resistance, oxidant-antioxidant status, endothelial dysfunction, lipid metabolism and their contribution to the risks of cardiovascular disease in women with PCOS. Fifteen women with PCOS and 17 healthy women were included in this case-control study. Nitric oxide (NO), endothelin-1 (ET-1), malondialdehyde (MDA), Apo A1, Apo B, small, dense LDL cholesterol (sdLDL-C), lipid levels and paraoxonase 1 (PON1) activity were measured in serum/plasma obtained from study groups. Insulin resistance (HOMA index - Homeostasis Model Assessment) and serum sex hormone profiles were also evaluated. Significantly decreased NO levels and PON1 activities, but increased MDA, ET-1 and sdLDL-C were found in PCOS patients compared to those of controls. Serum MDA, ET-1, HOMA and sdLDL-C levels decreased and PON1 activity and NO levels increased significantly after the metformin treatment. There was a positive correlation between MDA and free testosterone (fT), ET-1 and fT; and a negative correlation between PON1 activity and fT. Insulin resistance, dyslipidemia, endothelial dysfunction and oxidative stress might contribute to the excess risk of cardiovascular disease reported in PCOS. Metformin seemed to decrease oxidative stress and improve insulin resistance, dyslipidemia and endothelial dysfunction in PCOS patients.