This study measured the antioxidant activity of follicular fluid (FF) in infertile patients and assessed its possible correlation between ovarian stimulation and pregnancy outcomes. Samples from 191 infertile patients undergoing in vitro fertilization-embryo transfer (IVF-ET) were determined by alpha-diphenyl-beta-picrylhydrazyl (DPPH) radical scavenging, reducing power, superoxide radical scavenging, beta-Carotene bleaching assay, ferrothiocyanate and thiobarbituric acid assays. The comparison between a positive IVF outcome and FF's antioxidant activity was also studied. The results showed FF had strong antioxidant activity, which equated to common antioxidants Vc and BHT (100 mu g/mL). Patients with endometriosis had less efficient antioxidant activity in FF than that of patients with tubal occlusion or polycystic ovary syndrome. In conclusion, this study detected, for the first time, the antioxidant activity of FF from patients undergoing an IVF and the FF exhibited strong antioxidant activity.