Background: Adequate and optimum endometrial development is required for a successful pregnancy to occur. Clomiphene citrate is well known for its negative impact on endometrial thickness. The long-acting phosphodiesterase 5 inhibitor, Tadalafil, was shown to improve endometrial growth in patients under Clomiphene citrate stimulation. Objective: To study the effect of Tadalafil on the endometrial thickness in Clomiphene citrate stimulated cycles as compared to clomiphene citrate alone and human menopausal gonadotropins. Study design: A randomized controlled study. Setting was at the infertility clinic of Cytogenetic and Endoscopy Unit, Zagazig University Hospital. Material and methods: The study included 236 patients who underwent a single cycle of ovulation induction and timed intercourse. Patients were divided into 3 groups: Controlled ovarian stimulation was done in group A by Clomiphene Citrate alone, in group B by Clomiphene Citrate with addition of Tadalafil and in group C by Human Menopausal Gonadotropins. Results: As regard the number of cases who got pregnant, we found that there was a statistically significant difference (P < 0.05) for patients in group B and C when compared with group A. Conclusion: This study showed that the pregnancy rate achieved with Clomiphene citrate/Tadalafil combination was comparable to that achieved by Human Menopausal Gonadotropins and significantly higher than with Clomiphene citrate alone. This was attributed to the improvement of endometrial thickness by Tadalafil in Clomiphene citrate stimulated patients.