Flexible GnRH Antagonist Protocol versus Progestin-primed Ovarian Stimulation (PPOS) Protocol in Patients with Polycystic Ovary Syndrome: Comparison of Clinical Outcomes and Ovarian Response Summary Polycystic ovary syndrome (PCOS) is one of the most common causes of infertility in women. Progestin-primed ovarian stimulation (PPOS) protocol, which used oral progestin to prevent premature luteinizing hormone (LH) surges in ovarian stimulation, has been proved to be effective and safe in patients with PCOS. The aim of the present study was to compare the efficacy of PPOS protocol with that of the traditional gonadotropin-releasing hormone (GnRH) antagonist protocol in patients with PCOS. A total of 157 patients undergoing in-vitro fertilization (IVF) or intracytoplasmic sperm injection (ICSI) were recruited into this study. The patients were divided into two groups by the stimulation protocols: the GnRH antagonist protocol group and the PPOS protocol group. There was no significant difference in the clinical characteristics between the two groups. Dose and duration of gonadotropin were higher in the PPOS protocol group. Estradiol levels on the day of human chorionic gonadotropin (hCG) administration were significantly lower in the PPOS protocol group. Fertilization rates and the number of good quality embryos were similar between the two groups. Remarkably, we found 6 patients with moderate ovarian hyperstimulation syndrome (OHSS) in the GnRH antagonist protocol group but 0 in the PPOS protocol group. A total of 127 women completed their frozen embryo transfer (FET) cycles. There were no significant differences between the two groups in terms of clinical pregnancy rate per transfer, implantation rate, first-trimester miscarriage rate and on-going pregnancy rate per transfer. To conclude, PPOS protocol decreased the incidence of OHSS without adversely affecting clinical outcomes in patients with PCOS.