OBJECTIVE: This study was performed to investigate the effects of pioglitazone, an insulin sensitizing agent, on insulin resistance, ovarian function and intraovarian stromal blood flow in patients with polycystic ovarian syndrome (PCOS). MATERIAL AND METHODS: Thirty patients with PCOS, aged 18~34 years, were recruited. Criteria for diagnosis of PCOS were as defined in 2003 Rotterdam consensus. They were treated for 6 months with pioglitazone at a dose of 30 mg/day orally. The hormonal blood profile, fasting serum glucose levels, a glycemic response to 75 g oral glucose tolerance test (OGTT), and an ovarian stromal artery (OSA) blood flow were assessed at baseline and after 6 months of treatment. RESULTS: Eighteen (60.0%) of 30 patients treated with pioglitazone demonstrated a spontaneous ovulation After pioglitazone treatment, fasting insulin concentrations, serum glucose levels after 75 g OGTT significantly decreased (p=0.001, p=0.04, respectively), and fasting glucose to insulin (G/I) ratio significantly increased (p<0.001). The pioglitazone treatment induced a significant reduction in serum LH, testosterone (T) and free T levels (p<0.001, p=0.02, p=0.002, respectively). The resistance index (RI) values of OSA significantly increased after treatment (p<0.001). In analyzing pioglitazone-treated patients according to their body mass index (BMI), nonobese group as well as obese group showed a significant improvement in fasting G/I ratio (p<0.01). The pioglitazone treatment induced a significant reduction in serum LH and free T levels in nonobese group (p<0.001, p<0.05, respectively) as well as obese group (p=0.001, p<0.05, respectively). The RI values of OSA significantly increased in both nonobese and obese groups after pioglitazone treatment (p<0.001, p=0.003, respectively). CONCLUSIONS: Pioglitazone could ameliorate the glycoinsulinemic metabolism, and this beneficial effects of this drug could improve the endocrine-reproductive condition associated with the decrease of ovarian stromal artery blood flow, in both nonobese and obese patients with PCOS.