RESEARCH QUESTION:What is the outcome of fertility-preservation treatments in women with endometrioma, especially those with endometrioma larger than 4 cm? DESIGN:Retrospective cohort study. Women with definitive diagnosis of ovarian endometriosis (by histology or ultrasound), who underwent fertility-preservation treatment in two IVF units between 2016 and 2021, were included. As some women cryopreserved oocytes and other embryos, the primary outcome was the number of metaphase II (MII) oocytes retrieved. RESULTS:Seventy-one women with ovarian endometriosis (OMA) underwent 138 fertility-preservation cycles. The median age of patients was 31 years. Forty out of 71 (56%) women underwent at least one surgery for OMA before fertility-preservation treatment. Multivariate analysis of each patient's first cycle was used. Women who underwent OMA surgery before fertility-preservation treatment had a 51.7% reduction (95% CI 26.1 to 68.5, P = 0.001) in the number of MII oocytes compared with women with OMA who did not undergo surgery. Among a subgroup who did not undergo surgery, those with an endometrioma larger than 4 cm had similar anti-Müllerian hormone concentration (2.6 ng/ml versus 2.1 ng/ml), number of oocytes retrieved (9 versus 9) and number of MII oocytes (7.6 versus seven 7) compared with women with an endometrioma of 4 cm or less. CONCLUSIONS:Discussing fertility-preservation treatment options with patients with OMA is recommended, especially if surgery is planned.