Background: Letrozole is as effective as clomiphene citrate for induction of ovulation in patients with PCOS. Adequate endometrial development is required for pregnancy to occur, and pregnancy rates were found to be higher when the endometrium reached at least 10 mm thickness. Objective: To study the effect of estradiol valerate on the endometrial thickness and subsequently the pregnancy rate in PCOS cases receiving letrozole for induction of ovulation as compared to letrozole alone. Study design: A randomized controlled study. Setting was at the infertility clinic of the Cytogenetic and Endoscopy Unit, Zagazig University Hospital. Material and methods: The study included 273 patients who underwent ovulation induction and timed intercourse. Patients were divided into 2 groups: controlled ovarian stimulation was done in group 1 by letrozole with addition of estradiol valerate and in group 2 by letrozole alone. Results: As regard the clinical pregnancy rate, we found a statistically significant difference between the 2 studied groups, with documented clinical pregnancy in 34 patients in group 1 compared to only 16 patients in group 2. Conclusion: This study showed that the pregnancy rate achieved with letrozole/estradiol valerate combination was significantly higher than with letrozole alone. This was attributed to the improvement of endometrial thickness by estradiol valerate.