Purpose: Endometriosis is a common gynecological disorder characterized by the presence of functional endometrial glands outside of the uterus. It is thought to occur through either retrograde menstruation into the peritoneal cavity or metastases through blood and lymphatic vessels. Various radiographic studies including CT scan, MRI, transvaginal ultrasound, and rectal ultrasound have been used to detect peritoneal implants, but definitive diagnosis requires laparoscopy and biopsy. Endometriosis of the gastrointestinal tract most commonly involves the rectosigmoid colon due to its close proximity to the reproductive organs. Here we describe a rare case of endometriosis involving the appendix. The patient is a 56 year-old healthy woman with a history of prior hysterectomy and bilateral salpingo-oophorectomy for endometriosis who was found to have a 1 cm polypoid mass at the appendiceal orifice on screening colonoscopy. Endoscopic ultrasound of the mass with the miniprobe revealed a 10 × 7 mm anechoic structure suggestive of a cystic lesion. Biopsies of this lesion were nondiagnostic. A subsequent CT scan of the abdomen revealed a 9–10 mm area of decreased attenuation adjacent to the cecum, possibly representing the lesion of interest. The patient otherwise was asymptomatic and denied any abdominal pain, weight loss, night sweats, hematochezia, or diarrhea. The patient was referred to a colorectal surgeon, as a mucocele of the appendix was suspected. The patient underwent laparascopic right hemicolectomy several months later and the final pathology of the resected specimen revealed endometriosis. Appendiceal endometriosis is a rare phenomenon with a reported prevalence of 0.054%. Many cases are asymptomatic and pass undiagnosed. However, symptoms of acute appendicitis, chronic right lower quadrant abdominal pain, cecal intussusception, and intestinal perforation have been reported. Diagnosis is difficult as endometriosis of the gastrointestinal tract mimics other disorders such as irritable bowel syndrome and requires histological demonstration of ectopic endometrial glands and stroma. Methods: N/A Results: N/A Conclusion: N/AFigure