Objective: the aim of this work is to compare the effect of Letrozole versus Clomiphene citrate (CC) in induction of ovulation in Polycyctic ovarian syndrome (PCOS) patients. Patients and methods: The study was conducted in Al Maadi Military Hospital at the department of Obstetrics and Gynecology (out-patient infertility clinic). The study is a prospective randomized study consisting of 100 Egyptian patients diagnosed as having PCOS. Patients were divided into two groups with randomization

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... and allocation concealment: Group (A): includes 50 females taking Letrozole 5mg tab orally once daily started in day 3 to day 7 of menstrual cycle for 3 cycles. Group (B): includes 50 females taking Clomiphene Citrate 100mg tab orally once daily started day 3 to day 7 of menstrual cycle for 3cycles. Trans-vaginal ultrasound was done from day 9-11 of the cycle to document ovulation rate, endometrial thickness and number of follicles and from day 14 (mid cycle day) to evaluate the target size of the follicle (18-22mm) and to assess the need to HCG administration. Results: Letrozole is significantly higher regarding endometrial thickness (Let 9.2mm Vs CC 8.0mm) and pregnancy rate (Let 48% Vs CC 28%), while Clomiphene Citrate (CC) is significantly higher regarding number of follicles (48%) as it produces multiple number of follicles compared to Letrozole (26%) which produces mono-follicle only. Both drugs are significantly similar regarding the ovulation rate. The study concluded that Letrozole can be used as first line therapy for induction of ovulation in polycystic ovary syndrome patients. Conclusion: The pharmacodynamics of Letrozole (does not deplete ERs, short half-life, keeps intact hypothalamic pituitary axis) ensures improved endometrial thickness and cervical mucus and monofollicular ovulation. Therefore, these factors may lead to a higher pregnancy rate and greater likelihood of singleton pregnancy.