Objective: To determine the efficacy of letrozole in suppressing estradiol levels during ovarian stimulation in cancer patients. Methods: A retrospective chart review of cancer patients undergoing ovarian stimulation for fertility preservation between 2014-2019 at a private university-affiliated fertility clinic in Canada was conducted. Ovarian stimulation was completed with no letrozole (Group A, n = 10), and adjuvant daily letrozole use at 5.0 (Group B, n = 34) or 7.5 mg (Group C, n = 61). The primary outcomes were peak estradiol levels and oocyte yield. ANOVA with a post hoc two-tailed t-test assuming equal variance was utilized as a statistical method. Result(s): Patient age and AFC count were not different between groups. The yield of mature eggs was not different at each letrozole dose; 9.2 ± 6.0, 13.9 ± 6.5 and 12.7 ± 7.2 for Groups A to C respectively (p = 0.18). Mean estradiol levels(pmol/L) were reduced in a dose-dependent manner; 7432 ± 4553 for Group A, 2072 ± 1656 for Group B, and 1445 ±1238 for Group C (A vs. C, p vs. C, p Conclusion(s): The use of letrozole during ovarian stimulation for oocyte and embryo cryopreservation in cancer patients can maintain physiologic estradiol levels, while ensuring satisfactory oocyte and embryo yield. Letrozole can, therefore, minimize the theoretical risk of stimulating residual and metastatic diseases, while still optimizing future fertility outcomes.