Objectives: To compare the induction of ovulation and pregnancy with letrozole and clomiphene and combination therapy in polycystic ovary syndrome. Design: Cohort prospective intervention.setting: Shahid Akbar Abadi. Inclusion criteria: women with a history of infertility for more than one year and which were diagnosed with anovulation and polycystic ovary syndrome in 2003 based on the revised criteria. Exclusion criteria: women more than 40 and less than 20 years old, pipes anatomical problems approved by HSG, partner impaired sperm analysis based on a modified World Health Organization criteria, such as hypo- or hyperthyroidism; endocrine problems, Hyperprolactinemia; karyotype abnormalities. Intervention: 450 patients that were randomized with non-probabilistic methods available are divided into three groups of 150. Group A, B, C, respectively, treated with 100 mg clomiphene, letrozole 5 mg and 100 mg clomiphene citrate with letrozole 5 mg from the third day of menstural cycle for 5 days. Transvaginal sonography will be performed in 11th day of menstural cycle to evaluate of ovolation. The number of mature follicles, endometrial thickness, the presence of pregnancy, multiple pregnancies and abortions will be recorded under 20 weeks. The main outcome variables: the presence of ovulation, endometrial thickness, the presence of pregnancy, the number of mature follicles.