Preoperative diagnosis of appendiceal mucocele is difficult. We report one case that was diagnosed before surgery. The patient was a 68-year-old man who complained of a mass in the right lower quadrant of the abdomen. Ultrasonography disclosed a hypoechoic bilobular cystic mass approximately 12 cm in diameter. Tha mass had a fine-grained echoic pattern and posterior echo enhancement in the ileocecal region. The lesion was seen to be connected to the appendix and was believed to be a cystic lesion with mucinous fluid. It was diagnosed as an appendiceal mucocele. Results of other examinations could not help establish the diagnosis. At laparotomy, a fist-sized tumor was found attached to the appendix, and ileocecal resection was performed. The tumor was pear-shaped and contained a jellylike substance. Histologically, this lesion was diagnosed as a mucinous cystadenocarcinoma. Abdominal ultrasonography was the most useful of the preoperative examinations carried out to diagnose this lesion. Appendiceal mucocele should be kept in mind when performing routine ultrasonographic abdominal examination.