Oocyte and/or embryo vitrification after controlled ovarian stimulation (COS) represents the most established method of fertility preservation (FP) before cancer treatment. However, traditional COS regimens are associated with supraphysiologic serum estradiol and are therefore not recommended in estrogen-sensitive diseases such as breast cancer (BC). To protect the patients from the potential deleterious effects of elevated estrogen levels during COS for FP, protocols using aromatase inhibitors (letrozole) were developed. The present study aims at investigating whether COS with letrozole supplementation (COSTLES) modifies ovarian response in BC patients.