We hypothesized that the administration of rosiglitazone, an insulin-sensitizing agent of the thiazolidinedione class, would improve the ovulatory dysfunction, hirsutism, hyperandrogenemia, and hyperinsulinemia of polycystic ovary syndrome (PCOS) patients. Forty women with PCOS and impaired glucose tolerance test (IGT) were randomly assigned to the 8-month treatment with rosiglitazone at either 2 mg/day or 4 mg/day. We compared changes in ovulatory function, hirsutism, hormonal levels (total and free testosterone, estradiol, estrone, androstenedione, LH and FSH), and measures of glycemic parameters (fasting and post-challenge levels of glucose and insulin, HOMA-IR, hemoglobin A1c), between the study groups. The patients' baseline characteristics were similar across all treatment arms. Fifteen of 20 women in the 2 mg group and 19 of 20 women in the 4 mg group achieved normal glucose tolerance; 14 of 20 women in the 2 mg group and 17 of 20 women in the 4 mg group achieved ovulatory menses at the end of the study period. The decreases of free testosterone levels were better in the 4 mg group than the 2 mg rosiglitazone group (–1.89 ± 0.35 pg/ml vs. –2.21 ± 0.39 pg/ml; P<0.01). There were neither any serious adverse events nor any liver enzyme elevations in our study patients during the treatment period. This study demonstrated that rosiglitazone improves the ovulatory dysfunction, hirsutism, hyperandrogenemia, and insulin resistance of PCOS in a dose-related fashion, with minimal adverse effects. This drug may be a good choice for lifetime treatment of patients with PCOS, especially for the ones who failed to show satisfactory results in metformin therapy.