Objective To determine whether ileocaecal endometriosis (ICE) is a marker for low rectal endometriosis (LRE) severity.Design Retrospective cohort study. Setting France.Population and sample Analysis of 375 colorectal resections performed in women undergoing complete surgery for LRE from January 1995 to December 2015 in a university centre for endometriosis.Methods Univariate and multivariate analysis of anatomical, postoperative clinical, and long-term outcomes according to presence of ICE.Main outcomes and measures Mean number and type of deep infiltrating endometriosis (DIE) lesions, the existence of an associated endometrioma, and mean total American Society for Reproductive Medicine (ASRM) score.Results The prevalence of ICE was 25.6%. Primary end-point data showed that women with ICE had a significantly higher adjusted number of DIE lesions (OR = 1.43, 95% CI 1.02-3.03; P = 0.048), higher prevalence of endometriomas (OR = 1.91, 95% CI 1.04-3.51; P = 0.044), more associated DIE sigmoid lesions (OR = 2.12, 95% CI 1.07-3.91; P = 0.025), and a higher mean total ASRM score (OR = 2.07, 95% CI 1.12-4.14; P = 0.025). Women with ICE resected during the surgical procedure for LRE did not have more adverse postoperative clinical outcomes than ICE-negative patients.Conclusion Ileocaecal endometriosis was significantly associated with greater LRE severity. In a complete surgical resection strategy, combining resection of ICE and LRE did not appear to increase postoperative rates of complications, morbidity or recurrence, nor did it seem to impair long-term clinical outcomes.Keywords Complete surgery, deep infiltrating endometriosis, disease severity, ileocaecal endometriosis, low rectal endometriosis.Tweetable abstract In women with low rectal endometriosis, 25% have an associated ileocaecal location that is a marker for severity.