ASSOCIATED with the constant progress of medical therapy, there have developed newer, more accurate, and valuable techniques for diagnosis. This development is well exemplified in the field of retroperitoneal tumors. Tumors of this region may consist of adrenal cortex, medullary substance, or embryonal rests of adrenal tissue. The renal parenchyma and pelvis, as well as the ureter, produce the largest group of tumors in the retroperitoneal space. Neoplasms originating otherwise than from the above organs, but in the locale under discussion, may be any one of a great number of exotic tissue growths. These primary tumors1 may arise from vascular, neurological, lymphatic, or muscular cytoplasm, or even from remnants of the embryonic urogenital ridge.2 In addition, extravisceral retroperitoneal tumors may be present which are not neoplastic but are benign lesions, such as an aortic aneurysm or a cyst; extrarenal adrenal neoplasms may be metastatic in nature, but they