The retroperitoneum and the pelvis are difficult areas to examine by conventional radiographic means. Pelvic ultrasound can distinguish cystic from solid masses, but is poor in defining tissue planes. Computed tomography (CT) easily detects calcifications, is rarely affected by overlying bowel gas, and usually demonstrates the mass with good definition of tissue planes. Although less accurate than ultrasound in distinguishing cystic from solid masses, CT is superior for demonstrating contiguous anatomical structures such as the rectum, bony pelvis, vertebral bodies, abdominal aorta, and inferior vena cava. Overlying bowel gas often precludes obtaining a diagnostic ultrasound examination of the retroperitoneum. In addition, ultrasound is unable to reproducibly demonstrate soft tissue relationships in the retroperitoneum as does CT. Both CT and ultrasound are capable of providing diagnostic information. Ultrasound can more easily distinguish cystic from solid masses, but CT may be better for determining the extent of disease. CT should become the procedure of choice for evaluation of the retroperitoneum.