背景：子宫内膜异位症(endometriosis, EMS)是指子宫内膜组织(腺体和间质)异位至子宫体腔外生长。最常见于25~45岁育龄期女性，属于一种良性疾病，却有类似恶性肿瘤的远处转移和种植能力。按组织发生和临床表现不同可分为腹膜型、卵巢型、深部浸润型及其他部位的内异症4种。其中深部浸润型又可分为4型，第4型浸润至肠道，称为肠型子宫内膜异位症，直肠和乙状结肠是最易受累的部位，发生率约3.8%~37%，其次是小肠和盲肠。直肠、乙状结肠EMS患者轻则无明显临床症状，重者可引发腹痛、排便习惯改变或周期性便血等非特异性消化道症状，临床鉴别较困难，易误诊为肠道肿瘤。病例介绍：本文中一例病人即出现明显的便血症状后来我院就诊。两次内镜检查均考虑直肠肿瘤，行手术治疗后明确为直肠子宫内膜异位症。结论：结直肠子宫内膜异位症是一种极易与直肠肿瘤混淆的一种疾病，诊疗过程中，要详细询问病史及既往史，便血与月经周期的关系。术前行完善全腹CT及肠镜检查，必要时于超声内镜活检，仍不能明确诊断时，建议腹腔镜检查。 Endometriosis is defined as a disease of endometrial-like tissue outside the uterus. It is a benign disease for women of reproductive age 25~45 years, but has the capacity for distant metastasis and implantation similar to malignant tumors. There are four types of endometriosis: peritoneal type, ovarian type, deep infiltrative type, and endometriosis of other sites, depending on tissue occurrence and clinical manifestations. The fourth type infiltrates into the intestine and is called intestinal endometriosis. The rectum and sigmoid colon are the most susceptible sites, with an incidence of 3.8% to 37%, followed by the small intestine and cecum. Patients with EMS of the rectum and sigmoid colon may have no obvious clinical symptoms, while those with severe symptoms may have non-specific gastrointestinal symptoms such as abdominal pain, changes in bowel habits, or periodic blood in the stool, which are difficult to distinguish clinically and can be misdiagnosed as intestinal tumors. Case presentation: In this paper, a patient presented with obvious symptoms of blood in the stool and later visited our hospital. Both endoscopies considered rectal tumors, and after surgical treatment it was clearly rectal endometriosis. Conclusion: Colorectal endometriosis is a disease that can be easily confused with rectal tumors, and a detailed history of the disease and its past, as well as the relationship between blood in the stool and the menstrual cycle, should be asked in the course of treatment. Full abdominal CT and colonoscopy were performed preoperatively, and endoscopic ultrasound biopsy was present if necessary, but still no definite diagnosis was made, laparoscopic examination was recommended.