To evaluate oxidative stress indices in follicular fluid (FF)by a novel thermochemiluminescence (TCL) assay and investigate the correlation between TCL and IVF cycle parameters. Prospective, crosssectional study. IVF Unit of an Obstetrics and Gynecology Department in a universityaffiliated hospital. One hundred eightynine women undergoing consecutive IVF treatment cycles during 2001. After oocyte retrieval, pooled FF was centrifuged and the supernatant was tested in the TCL assay. Maximal serum E2 levels, number of gonadotropin ampoules, retrieved oocytes, mature oocytes, fertilization and cleavage rates, number of available embryos and cryopreserved embryos, and clinical pregnancy rates and correlation with TCL indices. The TCL curve slope of FF positively correlated with maximal serum E2 levels, number of mature oocytes, and number of cleaved embryos and was inversely correlated with the womens ages and the number of gonadotropin ampoules. Follicular fluid TCL amplitude at 50 seconds ranged from 294 to 711 cps, but all pregnancies (n = 50; 28.1%) occurred within the range of 347-569 cps. With 385 and 569 cps as cutoff levels for the occurrence of pregnancy, the negative predictive value beyond this range was 96%and the positive predictive value within this range was 32%. The TCL results may reflect the agerelated increase in free radical activity and is associated with parameters of ovarian responsiveness and IVF outcome. A certain threshold of oxidative stress may be required for the occurrence of conception in IVF. TCL is a potential tool to evaluate, treat, and monitor antioxidant therapy in IVF treatments. To evaluate oxidative stress indices in follicular fluid (FF) by a novel thermochemiluminescence (TCL) assay and investigate the correlation between TCL and IVF cycle parameters. Prospective, crosssectional study. IVF Unit of an Obstetrics and Gynecology Department in a university أصيل hospital. One hundred eightynine women undergoing consecutive IVF treatment cycles during 2001. After oocyte retrieval, pooled FF was centrifuged and the supernatant was tested in the TCL assay. Maximal serum E2 levels, number of gonadotropin ampoules, retrieved oocytes, mature oocytes, The TCL curve slope of FF positively correlated with maximal serum E2 levels, number of mature oocytes, and number of cleaved embryos and was inversely correlated with the women ages and the number of gonadotropin ampoules. Follicular fluid TCL amplitude a With 385 and 569 cps as cutoff levels for the occurrence of pregnancy, the negative predictive value beyond 294 to 711 cps, but all pregnancies (n = 50; 28.1%) occurred within the range of 347-569 cps. this range was 96% and the positive predictive value within this range was 32%. The TCL results may reflect the agerelated increase in free radical activity and is associated with parameters of ovarian responsiveness and IVF outcome. A certain threshold of oxidative stress may be required for the occurrence of conception in IVF. TCL is a potential tool to evaluate, treat, and monitor antioxidant therapy in IVF treatments.