ObjectiveTo investigate whether the estradiol/oocyte ratio (E-2/O) can predict ovarian response and pregnancy outcomes. MethodsThis is a retrospective cohort study on 1838 individuals undergoing in vitro fertilization with a depot gonadotropin-releasing hormone agonist (GnRH-a) long protocol. The patients were grouped by the quartiles of E-2/O: <131.2, 131.2-178.0, 178.0-242.9, and >242.9 pg/mL. ANOVA and chi (2) test with the Bonferroni adjustment were used for statistical analysis. ResultsWith the rise of E-2/O, the MII oocyte rate increased, whereas the number of oocytes retrieved decreased. The number of MII oocytes, fertilization rate, and the number of high-grade embryos were similar among the four groups. There were no differences in live-birth rate, clinical pregnancy rate, ectopic pregnancy rate, and miscarriage rate, but there was a trend towards a higher miscarriage rate in the patients with E-2/O between 131.2 and 242.9 pg/mL. ConclusionE(2)/O indicates the percentage of MII oocytes, but not the quality of an individual follicle. E-2/O does not predict live birth in a depot GnRH-a long protocol; however, there is a trend towards a higher risk for miscarriage in the patients with E-2/O between 131.2 and 242.9 pg/mL. Estradiol/oocyte ratio predicts the MII oocyte percentage, but not the quality of an individual oocyte or pregnancy outcomes.