Introduction: Frozen embryo transfer is associated with a higher rate of live birth and a lower risk for ovarian hyperstimulation syndrome in women with polycystic ovary syndrome (PCOS) compared with fresh embryo transfer. The aim of this study is to assess the optimal endometrial preparation protocol for women with PCOS undergoing frozen embryo transfer., Material and Methods: We conducted a historical cohort analysis of 1720 women with PCOS who underwent the "freeze-all" strategy between August 2014 and August 2017 because of their high risk for ovarian hyperstimulation syndrome. Three endometrial preparation protocols were used: natural cycle (NC; n = 191), which relies on the dominant follicle to secrete estrogen that then promotes endometrial growth; ovarian stimulation (OS; n = 96), which induces follicle growth using low doses of human menopausal gonadotropin; and hormone replacement (HRT; n = 1433), which uses exogenous estradiol to promote endometrial growth. The primary outcome was live birth., Results: For women who received a single embryo transfer, the live birth rates for the NC, OS, and HRT groups were 62.4%, 65.0%, and 52.2%, respectively. The live birth rate in the HRT group was significantly lower than that seen in the OS and NC groups (P = .009). The clinical pregnancy rates of the three groups were 72.3%, 73.8%, and 64.9%, respectively; this difference did not reach statistical significance (P = .071)., Conclusions: The rate of live birth with the NC and OS regimens was higher than with the HRT protocol in women with PCOS who undergo single-blastocyst frozen embryo transfer., (© 2020 Nordic Federation of Societies of Obstetrics and Gynecology.)