OBJECTIVE: To investigate the effects of pioglitazone on controlled ovarian stimulation (COS), IVF outcomes, and follicular fluid (FF) cytokine concentrations in patients with polycystic ovary syndrome (PCOS). METHODS: Eighty-six infertile patients with PCOS resistant to clomiphene citrate were randomized to receive pioglitazone (30 mg/day) or placebo on the starting day of oral contraceptive (OC) pretreatment, followed by an IVF protocol using a GnRH antagonist. Pioglitazone or placebo was administered once daily from the starting day of OC to the day of hCG injection. RESULTS: Total dose and days of recombinant follicle-stimulating hormone administered, and the numbers of retrieved and mature oocytes, were significantly lower in the pioglitazone group than in the control group. FF tumor necrosis factor-alpha (TNF-alpha) and interleukin-6 (IL-6) concentrations at oocyte retrieval were also significantly lower in the pioglitazone group. The clinical pregnancy rate was higher and the incidence of severe ovarian hyperstimulation syndrome was lower in the pioglitazone group, but the differences were not statistically significant. CONCLUSION: Pioglitazone reduces FF TNF-alpha and IL-6 levels, and may improve ovarian response to COS in patients with PCOS.