Polycystic ovary syndrome is a common heterogeneous endocrine disorder in reproductive-age women, with prevalence around 4-12%. The present study was performed to investigate whether progesterone-induced endometrial bleed before ovulation induction affects pregnancy in patients with PCOS who underwent intrauterine insemination (IUI) treatment. A total of 241 IUI cycles were retrospectively analyzed. Patients enrolled in this study underwent ovulation induction with IUI treatment from Jan. 2011 to Dec. 2012. The study group consisted of 184 cycles with progesterone-withdrawal bleed before ovulation induction. The control group included 57 cycles with spontaneous menses. The clinical characteristics, ovulation induction parameters and IUI outcomes, such as pregnancy rate and live birth/ongoing pregnancy rate, were compared between the two groups. We found that patients in induced shedding group had thinner peak endometrium in ovulation induction cycles. Additionally, the ratio of peak endometrial-thickness to baseline endometrial-thickness was lower in induced menses patients. However, the pregnancy rate and live birth/ongoing pregnancy rate per cycle were similar with the control group. Excluding the peak E 2 level, peak E-2/number of follicles > 15mm and peak endometrial-thickness/baseline endometrial-thickness, no differences were found in ovulation induction or IUI results between patients used Letrozole or Clomiphene Citrate. In patients undergoing administration with Letrozole, those taking progesterone had thinner endometrium and lower peak endometrial-thickness/baseline endometrial-thickness. However, the pregnancy rate and live birth/ongoing pregnancy rate were not statistically different from patients with spontaneous menses. In conclusion, our study showed that progesterone exerted a negative effect on endometrial development, which seemed to be associated with reduced pregnancy results in ovulation induction with IUI cycles.