Background: polycystic ovary syndrome (PCOS) is the major cause of anovulatory infertility. Letrozole and other aromatase inhibitors (AIs) have been introduced as a new treatment option could challenge clomiphene citrate (CC) for ovulation induction in this group. Objective: to compare the efficacy of short vs extended courses of letrozole therapy for ovulation induction in PCOS women. Patients and Methods: this is prospective randomized comparative study done on 60 patients attended El-Hussein Hospital and previously diagnosed as having PCOS, based on the revised Rotterdam criteria (2003). Patients were divided randomly into 2 treatment groups, group I, short letrozole therapy (30 patients, received 5mg of letrozole from 1st to 5th day of menstrual bleeding) and group II, long letrozole therapy (30 patients, received 2.5mg of letrozole from 1st to 10th day of menstrual bleeding). All patients of both groups monitored 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. Intercourse were advised for 24-36h after HCG injection. Serum HCG and trans-vaginal ultrasound were done 2 weeks after HCG injection in the absence of menstruation for diagnosis of pregnancy. Results: in long letrozole therapy, total number of follicles, mean number of mature follicles, endometrial thickness, percentage of patients ovulated after treatment, occurrence of pregnancy were greater when compared to results of short letrozole therapy. Conclusion: long letrozole therapy can produce more mature follicles and subsequently more pregnancies than short letrozole therapy.