['To examine the use of the aromatase inhibitor letrozole with FSH for ovarian stimulation in poor responders undergoing ovarian superovulation and IUI.', 'Observational cohort study as a prospective clinical trial in patients with unexplained infertility and a low response to ovarian stimulation with FSH.', 'Two tertiary referral infertility clinics associated with the Reproductive Sciences Division, University of Toronto.', 'Twelve patients with unexplained infertility undergoing IUI who received FSH alone in 25 prior cycles with poor response (less than three dominant follicles).', 'Patients were offered letrozole, 2.5 mg/day from day 3-7 of the menstrual cycle with FSH (50-225 IU/day) starting on day 5-7. hCG (10,000 IU) was given when two leading follicles were >/=2 cm followed by IUI.', 'Number of mature follicles (>1.8 cm), FSH dose, endometrial thickness, and pregnancy rate.', 'Improved response to FSH stimulation with letrozole co-treatment was evidenced by the significantly lower FSH dose associated with significantly higher number of mature follicles. During letrozole plus FSH stimulation cycles, clinical pregnancy was achieved in three cycles (21%).'] Does aromatase inhibition improve ovarian response to follicle-stimulating hormone in poor responders?