Objectives: To investigate the endometrial effects of letrozole in PCOS women with poor endometrial response (endometrium thickness 67 mm) to clomiphene citrate (CC) despite adequate ovulation, using the ultrasonographic markers of endometrial receptivity. Study design: Ambidirectional cohort study. Patients and methods: Sixty women with anovulatory PCOS having endometrial thickness less than 7 mm despite adequate ovulation with CC underwent ovulation induction with Letrozole (5 mg/day from cycle day 3 to 7) for one treatment cycle. Main outcome measures: Comparison of the endometrial thickness (ET) and pattern, uterine artery and spiral artery, resistance index (RI) and pulsatility index (PI) between the current letrozole and previous CC stimulated cycles. Results: In the current letrozole cycle compared with the previous CC cycles, there were significantly greater midcycle endometrial thickness (8.97 ± 1.32 vs. 5.7± 1.2, respectively; P < 0.05), multilayered endometrial pattern (93.33% vs. 50%, respectively; P