A 33-year-old female patient with ulcerative colitis was referred to our outpatient clinic in January 2008 with right lower abdominal pain without bloody diarrhea. Colonoscopy found mild proctosigmoiditis and a submucoal tumor with a maximal diameter of 5 cm in the cecum. Computed tomography revealed a large, hypodense, cystic cylindrical structure extending to the pelvic space. For severe pain, she underwent partial resection of the cecum including the tumor in March 2008. Intraoperatively, the vermiform appendix was swollen like a sausage and compressing the cecum, which accounted for what appeared to be a submucosal tumor like a volcano by endoscopy. Lymphadenectomy was not performed because malignancy was not suspected. In the surgical specimen, the vermiform appendix was spindle-shaped and contained a large quantity of viscous liquid. Postoperative pathological diagnosis was mucinous cystadenoma, and no cancer cells were present in the viscous liquid within the vermiform appendix. The patient left the hospital 7 days postoperatively, and her colitis remains in remission without any complications.