Research question: How does the number of oocytes used affect the cumulative live birth rate in endometriosis patients who had their oocytes vitrified for fertility preservation (FP)? Design: Retrospective observational study including data from 485 women with endometriosis who underwent FP from January 2007 to July 2018. Survival curves and Kaplan-Meier plots were used to analyse the cumulative live birth rate (CLBR) according to the number of vitrified oocytes used. Data were stratified according to age, stage of the disease and ovarian surgery prior to FP (operated vs. non-operated). Endometriosis curves were compared to plots developed using elective fertility preservation (EFP) patients as control group. Log-rank, Breslow and Tarone-Ware tests were used to compare the survival curves. Results: The CLBR increased as the number of oocytes used per patient rose, reaching 89.5% (95% CI=80.0-99.1) using 22 oocytes. Higher outcomes were observed in young women (≤35 y. vs. >35 y). In the younger group, the CLBR was 95.4% (95% CI=87.2-103.6) using ~20 oocytes vs. 79.6% (95% CI=58.1-101.1) in older women (P<0.05). No statistical differences were observed in overall calculations and according to age when the CLBR was compared between operated and non-operated women (NS). Comparable outcomes were also observed in stages I-II vs. III-IV (NS). The mean age was higher in EFP patients (37.2 ± 4.9 vs. 35.7 ± 3.7; P<0.05). The outcome was better in the endometriosis group as compared to EFP (P<0.05): a CLBR of 89.5% (80.0-99.1) vs. 59.9% (51.4-68.6) when 22 oocytes were used (P<0.05). However, the difference was milder when fewer oocytes were used in both groups. When comparisons were made between age-matching groups, no statistical differences were observed (NS). Conclusion: The probability of live birth increases as the number of oocytes used rises in patients with endometriosis, but better outcomes were observed among young women. Neither the stage of the disease nor prior surgical excision of ovarian endometrioma were related to success. No statistical differences in age matching groups were observed when comparing to EFP patients. The information provided herein may be of interest to both patients and treating physicians for counselling purposes.