An 80-year-old man presented to the emergency department with abdominal bloating and constipation. He had no lower urinary tract symptoms or gross hematuria but had lost 5 kg during the previous 8 months. The physical examination was notable for a distended abdomen with a mass that extended from the epigastrium to the pelvis and was nontender and dull on percussion. A distended bladder was suspected, and a urinary catheter was inserted. However, only a small amount of urine was drained, and the abdominal mass persisted. Computed tomography of the abdomen and pelvis, which was performed after the administration of intravenous contrast material, showed a large cystic mass (Panel A). During a surgical procedure, the mass, which measured 30 cm at its largest dimension, was found to involve the dome of the bladder but was separate from the bowels and mesentery (Panel B). Histologic analysis of the resected mass showed a urachal mucinous cystic tumor of low malignant potential, with no evidence of invasive carcinoma. Although some urachal tumors can have metastatic potential, this specific type is usually benign. The patient recovered well postoperatively, and at a follow-up visit 3 months later, he remained well with no further abdominal symptoms.