Carcinosarcomas are rare biphasic malignant neoplasms with an epithelial and a spindle cell component. We present a 62-year-old man with a history of noticeably abdominal distension, proved by surgery to be caused by carcinosarcoma of the renal pelvis and urinary bladder, occupying the entire left abdominal flank. We also illustrate the appearance of this rare entity on sonography and computed tomography.