[en] The solitary fibrous tumor is a rare spindle cell neoplasm of mesodermal origin. It usually arises in the pleura, though has recently been reported in the peritoneum and at nonserosal sites ; it is, however, extremely rare in the retroperitonum. The authors describe one case of a solitary fibrous tumor that occurred as a huge complex mass in the retroperitoneum of an adult. Preenhanced CT scans showed that the mass was sharply delineated from surrounding organs and that tumor density was the same as that of surrounding musculature. Peripheral Calcifications were noted, and postenhanced scans showed intense, gradual enhancement of solid portions of the mass, with nonenhanced cystic portions. Although not specific, solitary fibrous tumors must be included in the differential diagnosis of a large retroperitoneal tumor of complex consistency and with internal calcification