As one of the limited reference parameters for the appropriate timing of human chorionic gonadotropin (hCG) priming and embryo transfer (ET), peak serum estradiol (E-2) level and related parameters have been considered to be a possible marker of in vitro fertilization (IVF) outcomes. To our knowledge, few reports have investigated the correlation between the ratio of peak serum E-2 to the number of follicles >= 14 mm on the day of hCG administration (i.e., the E-2/fol. ratio) and the miscarriage rate (MR) in assisted reproductive cycles. In this study, a total of 1376 cycles were examined and grouped into quartiles by E-2/fol. ratio. The patient characteristics, controlled ovarian hyperstimulation (COH) performance, and IVF/ICSI results were compared between the four groups. Patients were further categorized as younger than 35 years of age or 35 years of age and older. The association between the E-2/fol. ratio and the implantation rate (IR) or MR was examined using the Mantel-Haenszel test for each group. We found that the E-2/fol. ratio correlated with the IR and MR for women younger than 35 years of age. There was a statistically significant increase in the IR with E-2/fol. ratio between 279.83 and 552.28 pg/ml, and women with an E2/fol. ratio > 552.28 pg/ml were more likely to suffer miscarriages. Our data support a role for cryopreservation of all embryos when E-2/fol. ratio exceeds 552.28 pg/ml for women younger than 35 years of age.