Objective. To evaluate the asymmetric dimethylarginine (ADMA) and nitric oxide (NO) levels in obese and lean patients with polycystic ovarian syndrome (PCOS) and find out their relation with hormonal and metabolic parameters. Methods. Twenty-two obese, 18 lean patients with PCOS and 11 obese, 24 lean healthy control patients were enrolled prospectively. Plasma ADMA and NO levels and arginine/ADMA ratio were evaluated on 3rd day of menstrual cycle after at least 10h overnight fasting. Results. Plasma ADMA, NO levels and arginine/ADMA ratio were similar in the groups. ADMA level did not correlate with the hormonal and metabolic parameters in patients with PCOS. However, NO correlated inversely with fasting insulin (r = -0.353, p = 0.041) and homeostasis model of insulin resistance (HOMA-IR) (r = -0.379, p = 0.027). Arginine/ADMA ratio also correlated inversely with fasting insulin (r = -0.339, p = 0.050). In multinomial regression analysis the risk of low NO was associated independently with high fasting insulin (OR = 1.19, 95% CI 1.0011.42, p = 0.049) and high HOMA-IR in patients with PCOS (OR = 2.26, 95% CI 1.034.98, p = 0.042). Conclusions. Insulin resistance may be the underlying mechanism of endothelial dysfunction through NO pathway in PCOS. © 2011 Informa UK, Ltd.