Gastroenterological (GI) symptoms are often reported by adults with endometriosis, leading to unnecessary diagnostic tests or complicated treatment. We investigated associations between endometriosis and irritable bowel syndrome (IBS) in adolescents and whether concurrent pain disorders affect these.We collected data within The Women's Health Study: Adolescence to Adulthood, which is a US longitudinal study of premenopausal females with and without endometriosis. Our study cohort included participants younger than 21 y enrolled from 2012-2018. Participants completed an extensive health questionnaire. Those with IBS based on self-reported diagnosis or meeting Rome IV diagnostic criteria were considered cases and those without IBS were controls. Subjects without concurrent gastrointestinal disorders or missing pain data (n=323) were included in analyses. We calculated adjusted odds ratios using unconditional logistic regression.More adolescents with endometriosis (54/224; 24%) had comorbid IBS compared to adolescents without endometriosis (7/99; 7.1%). The odds of IBS was 5.26-fold higher among participants with endometriosis than without (95% CI, 2.13-13.0). In girls with severe acyclic pelvic pain, the odds of IBS was 35.7-fold higher in girls without endometriosis (95% CI, 4.67-272.6) and 12-fold higher in girls with endometriosis (95% CI, 4.2-36.3), compared to no/mild pain. For participants with endometriosis, each 1-point increase in acyclic pain severity increased the odds of IBS by 31% (adjusted odds ratio=1.31; 95% CI, 1.18-1.47).In an analysis of data from a longitudinal study of girls and women with and without endometriosis, we found significant associations between endometriosis and IBS, and a linear relationship between acyclic pelvic pain severity and the odds of IBS. Increased provider awareness and screening for IBS and endometriosis will improve patient outcomes and increase our understanding of these complex disorders.