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Purpose. We investigated the yield of CT and MRI in the diagnosis and staging of 15 patients with retroperitoncal sarcomas; these lesions are often asymptomatic and need radical surgery to avoid local recurrences. Material and methods. April 1993, to September 1997, fifteen patients with retroperitoneal sarcomas were examined and then operated on. CT and CT-guided FNAB were always performed, while MRI was performed in 8 patients only. Because of the high risk of sampling errors, the bioptic specimens were immediately evaluated by a pathologist: a second sampling was required in 10 cases and a third sampling in 6 cases. MRI was performed with a .5 T system (GE Vectra); T1- and T2-weighted transverse images of the abdomen were acquired in all patients and additional coronal and sagittal images were acquired for each abnormal region. All patients underwent surgery and the pathologic diagnosis was compared with CT and MR findings. Resulte. CT always allowed accurate location of the lesions and identification of their components, especially fat deposits. The cytologic examination of FNAB samples allowed the diagnosis of sarcoma in 12 of 15 cases. CT results were compared with MR findings in 8 patients and the latter method had better spatial definition of the abdominal masses, particularly of vascular structures, which is important for surgical planning. Discussion. The present-day imaging techniques are very important to plan the surgical treatment of retroperitoneal sarcomas. In particular, CT has the advantage of high spatial resolution and excellent tissue contrast from abundant retroperitoneal fat tissue; it also permits the cytologic sampling of viable tumor tissue. However the bulk of such lesions often prevents CT from determining the tumor origin, in which case MRI provides better spatial resolution and vascular detailing, which helps surgical planning. Conclusions. Both CT and MRI are major tools in the diagnosis and staging of retroperitoneal sarcomas because they can accurately define the borders of large tumors and their relationships with surrounding organs. MRI has the advantage of characterizing the blood supply to the mass, but CT is better to guide FNAB.