The solitary fibrous tumor is a rare spindle cell neoplasm of mesodermal orgin. It usually arises in thepleura, though has recently been reported in the peritoneum, and at nonserosal sites; it is, however, extremelyrare in the retroperitonum. The authors describe one case of a solitary fibrous tumor occurred as a huge complexmass in the retroperitoneum of an adult. Preenhanced CT scans showed that the mass was sharply delineated fromsurrounding organs and that tumor density was the same as that of surrounding musculature. PeripheralCalcifications were noted, and postenhanced scans showed intense, gradual enhancement of solid portions of themass, 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.