Objective: To investigate the expression and role of oxidative stress markers in the serum and follicular fluid of patients with endometriosis. Materials and Methods: A prospective case-control study was conducted in 42 patients who underwent in vitro fertilization–embryo transfer (IVF-ET). They were divided into Group I: patients with endometriosis (n = 20) and Group II: patients with tubal factor infertility (n = 22). All patients underwent a long gonadotropin-releasing hormone (GnRH) agonist protocol for pituitary downregulation followed by controlled ovarian hyperstimulation. Level of reactive oxygen species (ROS), superoxide dismutase (SOD), and vitamin E (VE) were measured by enzyme-linked immunosorbent assay (ELISA). The results of IVF-ET between the two groups were compared. Results: The ROS levels in both serum and follicular fluid of the study group were significantly higher than in the control group. The serum levels of SOD and VE in the study group were significantly lower than those in the control group, but there was no difference in follicular fluid levels of SOD and VE between the two groups. Furthermore, the mature oocyte and fertilization rates in the study group were significantly lower than those of the control group. However, the levels of ROS, SOD, and VE in serum and follicular fluid were not significantly correlated with outcome following IVF-ET. Conclusion: Patients with endometriosis have increased oxidative stress, as well as lower mature oocyte rates and fertilization rates. Nevertheless, there is no evidence that the oxidative stress status is directly related to the outcome of IVF treatment.