Oxidative stress has a bidirectional role in the development and maturation of zygotes and embryos. Reduction-oxidation reactions and regulatory proteins, such as thioredoxin (TRX) and thioredoxin reductase (TRXR), are intimately involved in the regulation of oxidative stress. The aim of this study was to determine the levels of TRX mRNA and protein in ovarian follicles collected from women undergoing in vitro fertilization (IVF) and to assess these levels relative to follicle size, presence of oocytes, and responsiveness to superovulation. Follicular fluid (FF) and/or granulosa cells (GCs) from large and small follicles were collected at the time of ovum pick-up from 42 IVF patients enrolled in this study. We divided the patients into normal and poor responders (NR and PR, respectively) based on the serum estradiol levels on the day of human chorionic gonadotropin (hCG) administration. We also compared the TRX concentration in FF (FF-TRX) between oocyte-containing follicles (Oc+) and empty follicles (Oc-). The transcript levels of TRX, but not TRXR, were significantly higher in GCs derived from follicles collected from NR than PR, as determined by semi-quantitative RT-PCR analysis. In NR, the FF-TRX was significantly higher in Oc+ follicles than in Oc- follicles and also in large Oc+ follicles than in large Oc- follicles. Unlike NR, PR exhibited no positive association with elevated FF-TRX and presence of oocytes. Based on its collective anti-oxidative, cytoprotective, and cytokine-like properties of TRX, TRX is likely to be involved in the optimal growth and maturation of ovarian follicles and responsiveness to hyperstimulation.