Endometriosis is a common disease of women which occasionally causes serious symptoms in virtue of involvement of the distal large intestine. Two cases are reported, illustrating opposite ends of the spectrum of this disorder, one presenting with a complete large-bowel obstruction, and the other with periodical per-rectal bleeding. Endometriosis in these locations can closely mimic carcinoma, and from a consideration of our cases and the literature, those features which might help to distinguish one from the other are discussed. The treatment of endometriosis itself is briefly outlined. Either to avoid unnecessarily radical surgery, or to plan appropriate specific treatment at the time of surgery for the colorectal lesion, a preoperative diagnosis is desirable, but it is unlikely unless the clinician is aware of this unusual form of endometriosis.