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OBJECTIVE To compare the influence of dual suppression with the use of GnRH agonist plus aromatase inhibitor compared with suppression with the use of GnRH agonist alone or no suppression at all in patients with idiopathic recurrent implantation failure (RIF). DESIGN Retrospective cohort study. SETTING University-affiliated reproductive center. PATIENT(S) A total of 523 infertile women who failed two blastocyst transfers underwent a third frozen blastocyst transfer. Women with known endometriosis were excluded. INTERVENTION(S) A total of 204 subjects were not pretreated, 143 received 2 months of GnRH agonist (3.75 mg intramuscular leuprolide acetate monthly) only, and 176 received GnRH agonist and aromatase inhibitor (5 mg oral letrozole daily for 60 days). Demographic and stimulation information was collected and cycle outcomes reported. MAIN OUTCOME MEASURE(S) Clinical pregnancy rates. RESULT(S) Age, antral follicle count, basal FSH levels, duration of infertility, previous pregnancies, and full-term deliveries were similar (P>.05). Clinical pregnancy rates were higher among women who received GnRH agonist plus letrozole compared with women who received GnRH agonist only or women without pretreatment (63%, 42%, and 40%, respectively; P<.0001). Live birth rates were higher among women who received GnRH agonist plus letrozole compared with the other groups (56%, 36%, and 34%; P<.0001). No differences in pregnancy outcomes were noted between patients who did not receive pretreatment and those in the GnRH agonist only group. CONCLUSION(S) In patients with RIF, treatment with a GnRH agonist plus letrozole may improve live birth rates in subsequent cycles. We hypothesize that this improvement is due to alterations in the endometrium receptivity or treatment of undiagnosed endometriosis.