PURPOSE: To differentiate mature and immature ovarian teratomas, using CT findings. MATERIALS AND METHODS: The CT findings of ten mature ovarian teratomas (in one patient, bilateral) and ten which were immature were compared, using statistical analysis. images were evaluated for size, margins, architecture, contents (muralnodules, fat, calcification), septa, local invasion and distant metastasis. These findings were compared with pathologic findings. RESULTS: Of the ten mature tumors, nine were well defined and predominantly cystic ininternal architecture, and one was mixed. Mural nodules were found in six tumors, fat in all, distinct calcification in seven, and regular septa in three lesions. Of the ten immature tumors, eight had irregular margins, Seven were predominantly solid in internal architecture and irregularly enhanced, two were mixed, and one was mainly cystic. Fat was detected in five lesions, indistinct scattered calcification in six, irregular septa in three, and local invasion or distant metastasis in four patients. CONCLUSION: Compared with mature ovarianteratomas, those that are immature tend to show CT findings of marginal irregularity, solid mass with irregular enhancement, scattered indistinct calcifications, septal irregularity, local in vasion or distant metastasis. Our experience suggests that these findings may be helpful in differentiation of mature and immature ovarianteratomas.