PURPOSE: It is known that due to both their imaging and pathologic features, the accurate differentiation ofchondrosarcoma from chordoma is difficult. Through an analysis of MR and CT findings, this study aims to determinethe differential points between these two tumors. MATERIALS AND METHODS: In 21 patients, CT and MR imagingstudies of chordoma(n=12) and chondrosarcoma(n=9) at the base of the skull were retrospectively reviewed.Diagnosis had been established by histologic examination of surgically removed specimens. Eleven of the chordomaswere subclassified as 'conventional' and one as 'chondroid'; eight chondrosarcoma were 'conventional' andone was 'myxoid'. Four chordoma patients underwent CT and MR ; in six, only MR was performed ; and in two, onlyCT. Eight chondrosarcoma patients underwent both CT and MR, while in one, only CT was performed. All scans wereretrospectively evaluated for the location(midline/off-midline), direction of extension, margin and shape, bonydestruction and calcification, MR signal intensity and enhancement patterns of the tumors. Degree of calcificationwas graded from I to III. RESULTS: The epicenter of the mass of ten chordomas (83%) was midline, in twochondrosarcomas(22%), this was off-midline. Foci of calcification were observed in three of eleven chordomas(27%),and heavy calcification (grade III) was observed in one. In contrast to these findings, foci of calcificationswere observed in eight of nine chondrosarcomas (89%) and grade III calcification was observed in five. In bothtumors, calcification was coarse. Between the two kinds of tumor, there were no significant difference in MRsignal intensity and enhancement patterns, margin and shape, or direction of extension. CONCLUSIONS: Although MRand CT findings were similar in both types of tumor, location and degree of calcification may be features whichusefully distinguish chordoma from chondrosarcoma.