The aim of this study was to evaluate the relationship of oestradiol level on the day of hCG (peak E2)/oocyte ratio and the outcome of ART cycles. Of the patients who underwent IVF-ET, 600 normal and high responders to the first cycle of COH with gonadotropin releasing hormone (GnRH)-agonist were included in the study. Patients were designated into three groups based on peak E2/oocyte ratio (Group A: <100 pg/ml per oocyte, Group B: 100-200 pg/ml per oocyte, Group C: >200 pg/ml per oocyte). A comparison among groups was made regarding ovarian stimulation characteristics, fertilisation, implantation and pregnancy rates. After the division based on E2/oocyte ratio, in Group C, the number of oocytes retrieved, 2PN and M2 oocyte were statistically lower than both of the other two groups (p = 0.001, 0.001, 0.001, 0.045). HCG day E2 level was significantly different in all groups (p = 0.001), and fertilisation rate was meaningfully highest in Group C and lowest in Group A (p = 0.001). No difference existed among the three groups with respect to the number of embryos transferred and implantation rates. However, clinical pregnancy rate was significantly lower in Group A than others (p = 0.04). In ART cycles suppressed by GnRH-agonist, IVF outcomes are lower in patients with an E2/oocyte proportion of <100 pg/ml per oocyte.