The indication for surgical treatment of tumors of the retroperitoneum should not depend on findings of radiological examinations, because all radiographic procedures lack tissue specificity. Instead, it can only be determined histologically after an open biopsy or resection of the tumor, which should always be attempted. With the help of appropriate radiological examinations, a retroperitoneal mass will not only be detected, but the relationship to the adjacent anatomical structures will also be delineated. Therefore, valid information for planning the operation and estimating the morbidity and mortality can be provided for the surgeon.