Letrozole is an effective treatment for anovulatory infertile women. Letrozole appears not to have any adverse effects on the endometrium which is frequently associated with clomiphene citrate during ovulation induction [1, 2]. Many researchers have tried letrozole for ovulation induction in different methods [3–9]. Letrozole induced fewer mature follicles that can decrease multiple-pregnancy rate and risk of ovarian hyperstimulation syndrome. Therefore, the letrozole as the first-line drug of ovulation-induction agents in polycystic ovarian syndrome (PCOS) patients can be acceptable [10]. Segawa et al. have accepted letrozole and clomiphene to have the same effect in pregnancy outcome in PCOS patients although letrozole is not toxic and does not have any significant congenital anomaly associations [11–13]. In this study we have recommended letrozole in serial doses of 2.5, 5, and 7.5 mg in each cycle in clomiphene citrate-resistant infertile women with PCOS. The main interest of this paper is the assessment of efficacy and complication of letrozole in follicular size and number, pregnancy rate, abortion, endometrial thickness, and cumulative pregnancy rates in a clinical trial of clomiphene citrate-resistant patients.