Acute appendicitis occurs with such frequency that other lesions of this organ are often forgotten, overlooked, or not considered by the radiologist. This is particularly true when a soft-tissue mass is identified within the abdomen, in an unusual or atypical position for an appendiceal lesion. One such case is presented in this paper. Woodruff and MacDonald (3), in reviewing 146 cases out of approximately 43,000 appendectomies at the Mayo Clinic, found that simple mucoceles of the appendix may occur at almost any age. Apparently, however, they are more frequent in the fourth, fifth, and sixth decades of life, with a slightly higher incidence in women. Cystocarcinomas are less common than simple mucoceles of the appendix and are the result of malignant change in the wall of the benign mucocele. Mucoceles of the appendix are reputed to displace the cecum medially and may on occasion contain calcium in the wall or substance of the mass (1). A 49-year-old white male was admitted through the emergency room because of abdominal pain which had been present for one week. The patient stated that he had had lower abdominal pain, fever, and constipation about two months previously. He had also noticed firmness in the left flank, which he thought was possibly muscles enlarging as a result of exercise. Because of an enlarging abdomen he had his trouser waistline widened. His wife noticed his eyes were yellow one week before admission, and he observed that his urine had been brown. Physical examination revealed a well nourished patient. A large hard mass, estimated to measure 30 × 15 cm, occupied the left half of the abdomen, crossing the midline in the suprapubic region. It could not easily be demarcated by palpation, however. The liver edge was palpable 3 finger-breadths below the costal margin. X-ray examination of the abdomen performed on admission demonstrated a large soft-tissue mass in the left midabdomen (Fig. 1). The following day excretory urography showed no abnormality, but a barium enema disclosed lateral displacement of the medial wall of the descending colon (Fig. 2). On exploratory laparotomy, a large cystic mass which had omentum tightly adherent to its surface was excised. Pathologic examination revealed mucocele of the appendix with malignant change in the wall. The patient's postoperative course was uneventful, but after several months he died. At autopsy widespread metastases were found, including pseudomyxoma peritonei. Summary An unusual case of mucocele of the appendix with malignant change is presented. This is not a rare pathological entity and should be considered in the differential diagnosis of abdominal masses.