Background. The study was aimed at investigating the effects of ovarian high response during controlled ovarian hyperstimulation (COH) on implantation and pregnancy outcome in fresh IVF/ICSI cycles, and subsequent frozen-thawed embryo transfer (FET) cycles. Methods. An analysis of 1,196 cycles using a long protocol with GnRHa and rFSH was performed. A serum oestrial level (peak E2) was obtained on the day of hCG administration, and patients were grouped by peak E2 percentile distribution into 3 groups. Normal responder was set as cut-off concentrations between percentile (P)25 and P75 (Group A: 1,199–3,047 pg/ml, 595 cycles). Moderate high responders were classified as peak E2 between P75–P90 (Group B: 3,048–4,127 pg/ml, 180 cycles). For the high response group, the E2 cut-off concentration was set as P90 and above (Group C: ≥4,128 pg/ml, 119 cycles). Oocyte/embryo parameters and clinical outcomes were compared among the 3 groups in fresh cycles and subsequent FET cycles. Results. Comparisons between groups revealed no difference in the quality of oocyte retrieved and in fertilisation rates. Group C showed decreased trends in implantation and pregnancy rates compared with Group A, but statistical significance was reached only for the difference in implantation rates. Implantation and pregnancy rates in FET cycles were similar among the 3 groups. Conclusions. High serum estrogen levels were detrimental to implantation, but not to the quality of oocytes, which may be due to an adverse effect on endometrial receptivity in COH cycles.