Objective: To investigate the relationship between serum P levels on the day of hCG administration and pregnancy outcomes in different responders undergoing IVF. Design: Retrospective study. Setting: Teaching hospital. Patient(s): A total of 11,055 women who underwent their first IVF/intracytoplasmic sperm injection cycles and a subgroup of 4,021 women undergoing frozen-embryo transfer (FET) cycles. Intervention(s): Patients underwent IVF-ET with the long GnRH agonist protocol. The ovarian response was classified as high (>= 20 oocytes; n = 2,023), poor (<= 4 oocytes; n = 827), or intermediate (remaining cases; n = 8,205) according to the number of oocytes retrieved. Clinical outcomes of IVF-ET and FET cycles were analyzed according to plasma P levels. Main Outcome Measure(s): Ongoing pregnancy rates (PRs). Result(s): Ongoing PRs in fresh cycle were inversely associated with serum P levels on the day of hCG administration for all patients. Different P threshold concentrations were determined according to different ovarian response: We proposed a serum P level of 1.5 ng/mL as the threshold for poor responders, 1.75 ng/mL for intermediate responders, and 2.25 ng/mL for high responders. Our study does not show negative results for elevated P levels on oocyte performance in terms of fertilization, cleavage rate, or PR of FET cycles within different ovarian responses, offering no evidence for a detrimental effect of high P on oocyte quality. Conclusion(s): Elevated P levels on the day of hCG administration negatively influence PR regardless of different ovarian responses, although increased P threshold concentration is associated with better ovarian responses. The detrimental effect of P elevation on PR seems to be unrelated to oocyte quality in all responders. (Fertil Steril (R) 2012; 97: 1321-7. (C) 2012 by American Society for Reproductive Medicine.)