STUDY OBJECTIVE: To determine the characteristics of adolescent and young adult patients undergoing surgery for severe endometriosis METHODS: We analyzed the British Society for Gynaecological Endoscopy (BSGE) database, a multicenter database of research-consented patients undergoing excision of surgically confirmed deep endometriosis, from 2009 to 2021, which includes prospectively collected preoperative questionnaire data and intraoperative findings reported by the surgical team during laparoscopy. Patients were divided into adolescent (10-19 years), young adult (20-24 years), and older adult (≥25 years) groups. Preoperative questionnaires and intraoperative findings were compared between groups. RESULTS: The study included 14,670 patients from 104 specialist endometriosis centers, of whom 186 were adolescents and 892 were young adults. Young adults had worse premenstrual and menstrual pain, dyspareunia, dysuria, and bladder voiding difficulty than older adults, despite higher use of hormonal contraceptives (35.1% vs 17.7%, P < .001) and opioids (32.5% vs 27.8%, P = .004). Adolescent patients had lower prevalence of bowel (66.7% vs 81.1%, P < .001), ovarian (30.6% vs 51%, P < .001), deep sidewall (65.1% vs 78.1%, P < .001), and uterosacral ligament endometriosis (74.2% vs 85.0%, P < .001) compared with older adults. The same was true for young adults, who also had higher rates of superficial disease (79.7% vs 70.9%, P < .001) than older adults. Older adults had a higher number of pelvic sites affected by endometriosis than adolescents (6.0 vs 4.57, P < .001) and young adults (6.0 vs 4.9, P < .001). CONCLUSION: Adolescent and young adult patients undergoing endometriosis surgery report severe preoperative symptoms but have fewer anatomical sites affected and fewer deep endometriotic lesions than older adults. These findings highlight differences in lesion distribution that warrant further investigation into disease progression.