STUDY OBJECTIVE: According to a four-level scale of endometriosis surgical complexity, the 2021 American Association of Gynecologic Laparoscopists (AAGL) endometriosis classification system was externally and prospectively validated. And we explored the relationship between it, and clinical symptoms, CA125 and CA199 levels. DESIGN: Prospective, AAGL, validation, CA125. SETTING: The Affiliated Hospital of Qingdao University. PATIENT: Finally, a total of 164 patients were analysed. INTERVENTION: Intraoperative imaging (e.g. surgical video/picture) was analysed. For patients with positive pain symptoms, pain was rated from 0 to 10 using the visual analogue scale (VAS) scores. Preoperative serum CA125 andCA199 levelswere measured. MEASUREMENTS AND MAIN RESULTS: Weighted kappa coefficient analysis served to evaluate the consistencies across the AAGL staging system and ASRM system with the surgical complexity scale. The Mann-Whitney U test was employed to analyse the relationship with the AAGL staging system and CA125 and CA199 levels of patients. The weighted kappa score for the overall performance of the AAGL staging system and ASRM to predict AAGL level was 0.665 and 0.287, respectively. Our study showed the CA125 levels of patients having the AAGL stages Ⅲ/ Ⅳ disease exceed the stages Ⅰ / Ⅱ disease(p＜0.05). The CA199 levels of these two groups had no significant difference (p＞0.05). And compared with the patients having AAGL stages Ⅰ / Ⅱ disease, the dysmenorrhea, cyclic low back pain and deep dyspareunia of the patients having AAGL stages Ⅲ / Ⅳ disease had significant differences(p <
0.05). CONCLUSION: The 2021 AAGL endometriosis system assisted surgeons accurately evaluating the surgical complexity of endometriosis. Still, the CA125 levels and some clinical symptoms could reflect the surgical complexity of endometriosis.