Background: Polycystic Ovary Syndrome (PCOS) is one of the most common causes of an-ovulatory, 4-7% of women, The aim of the study is to compare the extended letrozole regimen with the classical one in treatment of CC resistant PCO patients, Subjects and Methods: 60 infertile, PCO and CC resistant women were divided randomly into 2 groups, group I, (short letrozole therapy) received 2.5mg twice daily from 2nd day of cycle for 5 days and repeated for 3 cycles (classical regimen)and group II, (long letrozole therapy) treated by 2.5mg once daily from 2nd day of cycle for 10 days and repeated for 3 cycles (extended regimen). Follicular growth and endometrial thickness monitoring, in all patients was performed regulary by trans-vaginal ultrasound. On day 10 of the cycles. HCG injection (5,000 IU IM) was given when at least one follicle measured ≥ 18mm followed by timed intercourse. Results: ovulation after treatment among group I (56.7%) compared to group II (63.3%) (P=0.598). Total number of follicles during stimulation was significantly greater in group I (6.51 ±0.78) vs. (4.2 ±0.81) in group II; (P=0.001). The mean ± SD of endometrial thickness at HCG injection among group I and group II was 8.4±1.76 mm and 8.83±1.45 mm respectively without significant difference between them (P=0.076). Pregnancy rate in group I was (13.3%) while (26.7%) in group II showing significant statistical difference. (P = 0.017). Conclusion: Treatment with long letrozole protocol can produce more mature follicles and subsequently increased pregnancy rate than the short letrozole therapy