Objective: To evaluate the impact of aromatase inhibitors as adjuvant treatment in IVF cycles on intraovarian androgens and cycle outcome. Design: Observational, pilot study. Setting: University-affiliated IVF unit. Patient(s): One hundred forty-seven low responder patients with a previous canceled IVF cycle; 71 patients were treated with letrozole 2.5 mg plus a high-dose FSH/hMG-antagonist regimen, and 76 patients were similarly treated but letrozole was not employed. Intervention(s): In vitro fertilization treatment with an antagonist FSH/hMG protocol with or without letrozole was administered during the first 5 days of stimulation; hormones were evaluated in both serum and follicular fluid. Main Outcome Measure(s): Number of oocytes retrieved, fertilization rate, implantation rate, and pregnancy rate; androstenedione, T,E2, and P values in serum and follicular fluid. Result(s): Letrozole-treated patients showed significantly higher levels of follicular fluid T and androstenedione (80.3 vs. 43.8 pg/mL and 57.9 vs. 37.4 mg/mL, respectively). Similarly, these patients had a higher number of oocytes retrieved (6.1 vs. 4.3) and a higher implantation rate (25% vs. 9.4% ) despite similar doses of FSH/hMG (3,627 vs. 3,804 IU). Conclusion(s): Adding 2.5 mg of letrozole to a high-dose FSH/hMG antagonist protocol increases intraovarian androstenedione and T concentration and improves IVF cycle outcome in poor responder patients. Objective: To evaluate the impact of aromatase inhibitors as adjuvant treatment in IVF cycles on intraovarian androgens and cycle outcome. Design: Observational, pilot study. Setting: University-affiliated IVF unit. Patient (s): One hundred forty-seven low responder patients with a previous canceled IVF cycle; 71 patients were treated with letrozole 2.5 mg plus a high-dose FSH / hMG-antagonist regimen, and 76 patients were treated treated with letrozole was not employed. Intervention (s): In vitro fertilization treatment with an antagonist FSH / hMG protocol with or without letrozole was administered during the first 5 days of stimulation; hormones were evaluated in both serum and follicular fluid. Main Outcome Measure (s): Number of oocytes retrieved, fertilization rate, implantation rate, and pregnancy rate ; androstenedione, T, E2, and P values ​​in serum and follicular fluid. Result (s): Letrozole-treated patients showed significantly higher levels of follicular fluid T androstenedione 0.3 vs. 43.8 pg / mL and 57.9 vs. 37.4 mg / mL, respectively). Similarly, these patients had a higher number of oocytes retrieved (6.1 vs. 4.3) and a higher implantation rate (25% vs. 9.4%) Conclusion (s): Adding 2.5 mg of letrozole to a high-dose FSH / hMG antagonist protocol increases intraovarian androstenedione and T concentration and improves IVF cycle outcome in poor responder patients.