Background <p>Fertility preservation is an important aspect of care for breast cancer patients. This study was designed to compare the ovarian function including ovarian reserve and ovarian stimulation response in breast cancer patients undergoing controlled ovarian stimulation (COS) for fertility preservation before chemotherapy, and oocyte donors as healthy women.</p> Methods <p>This historical cohort study enrolled 78 breast cancer patients who underwent COS for fertility preservation between April 2014 and March 2022 at the Infertility Center of Royan Institute, Tehran, Iran. Sixty-six healthy oocyte donors were included as a control group during the same time period. The inclusion criteria were female patients aged ≤ 35 years, with confirmed breast cancer with indication of chemotherapy, who were elected for ovarian stimulation before chemotherapy for oocyte retrieval. Women with the diagnosis of poor ovarian response, polycystic ovary syndrome, severe male factor infertility, endometriosis, and those who used oral contraceptives excluded from the study. Main outcome measure was retrieved total number of mature oocytes.</p> Results <p>The mean age was significantly higher and mean body mass index was significantly lower in the breast cancer group than in the oocyte donor group. There were also significant differences between groups in terms of hormonal profiles of luteinizing hormone and anti-mullerian hormone (AMH), gonadotropin starting dose, total dose of gonadotropin used and oocyte maturation rate. Based on the results, there was a significant correlation between Metaphase II and AMH in breast cancer group (<i>r</i> = 626; <i>P</i> &lt; 0.001).</p> Conclusion <p>It appears that patients with cancer experience diminished ovarian reserve prior to cancer treatment. Further prospective studies with larger sample sizes are recommended.</p> Clinical trial number <p>Not applicable.</p>