The present study was designed in order to: (a) compare ET-1 and ADMA levels, between women with PCOS (n=106) and healthy controls (n=30); (b) determine the effects of treatment with estrogens and anti-androgens on the hormonal features of PCOS, insulin resistance, ET-1 and ADMA levels. Women with PCOS were randomized in five therapeutic protocols: (I) 17beta-estradiol+cyproterone acetate 50mg; (II) conjugated estrogen+CA 50 mg; (III) ethinyl estradiole+CA 2mg; (IV) EE+CA 52 mg; (V) EE+desogestrel. In all women, gonadotropin, PRL, androgen, SHBG, insulin, glucose, ET-1 and ADMA levels were determined; in women with PCOS, testosterone, SHBG, ET-1 and ADMA levels were measured again after 3, 6, 12 months of treatment and insulin and glucose levels after 12 months. ET-1 and ADMA concentrations were higher in women with PCOS, and they were positively correlated with each other. ADMA levels were decreased and IR was increased with treatment. Treatment with synthetic estrogens (EE) resulted in a more pronounced increase in SHBG and a more pronounced decrease in FAI, compared to natural estrogens. Conclusively, PCOS is associated with endothelial dysfunction, which is ameliorated by the administration of estrogens and anti-androgens, independent of IR.