Retroperitoneal soft tissue tumors are frequently incidental findings on imaging tests as Computed tomography (CT) or Magnetic Resonance Imaging (MRI). Retroperitoneal soft tissue tumors are rare and therefore not common in daily radiological practice. Clinician and radiologist'skills to set retroperitoneal soft tissue tumors at presentation is crucial for a correct patient management. So far, several diagnostic algorithms have been proposed to assess retroperitoneal masses, which have not been validated by case histories (2-5). The aim of this article is to evaluate a new classification of retroperitoneal masses using CT and MRI. KEY WORDS: CT, Diagnosis, MRI, Retroperitoneum, Soft tissue sarcoma.