OBJECTIVE: The purpose of this study was to evaluate and compare the effects of metformin and rosiglitazone in overweight or obese women with polycystic ovarian syndrome. METHODS: Twenty Six overweight or obese patients with polycystic ovarian syndrome were randomly treated with either metformin (500 mg three times daily, n=13) or rosiglitazone (4 mg once daily, n=13) for 6 months. Hormonal studies were performed before and after treatment. Insulin resistances were calculated by computerized HOMA 2 Calculator v2.2. RESULTS: Testosterone decreased while SHBG increased after 6 months treatment in both metformin and rosiglitazone treatment groups. Fasting glucose decreased after metformin or rosiglitazone treatment. HOMA insulin resistance improved after treatment with either drug. There was no differences in hormonal changes and insulin resistance between 2 treatment groups. CONCLUSIONS: This study shows that metformin and rosiglitazone are effective in improving insulin sensitivity and ameliorating hyperandrogenism in overweight/obese polycystic ovarian syndrome women.