Though the role of reactive oxygen species (ROS) in female infertility has been a subject of rigorous research worldwide, there is inadequate information on the cut-off value of ROS in the oocyte microenvironment beyond which ART outcome may be adversely affected. An upper ROS level in follicular fluid (FF) samples of women undergoing IVF beyond which good quality embryo formation is unlikely, is established. ROS, lipid peroxidation and total antioxidant capacity were estimated. The upper cut-off ROS level beyond which viable embryo formation is not favorable was found to be approximately 107 cps/400 microl FF. This level, determined in women with tubal factor infertility, was further validated in women with endometriosis and PCOS and correlated with fertilization and pregnancy rate and embryo quality. Summarizing, a threshold level in FF has been established for the first time beyond which ROS may be considered toxic for viable embryo formation and pregnancy outcome.