PURPOSE: A tumor and mature ovarian cystic teratoma rarely coexist, but since it may affect treatment andprognosis, appropriate pre-surgical diagnosis is important. The purpose of this study was to evaluate thedifference in CT findings between a tumor coexisting with a mature ovarian cystic teratoma and a simple matureovarian cystic teratoma. MATERIALS AND METHODS: CT scans of seven cases of tumor coexisting with mature ovariancystic teratoma were reviewed. In each case, size, margin, nature, septation, solid portion, ascites, invasion,and metastasis were analysed. RESULTS: Coexistent tumors were histopathologically diagnosed as squamous cellcarcinoma in three patients, carcinoid in three, and fibrothecoma in one. In contrast with a simple matureteratoma, a tumor coexisting with a mature cystic teratoma developed in older patients and had a more solidportion, which showed contrast enhancement but did not show calcification or a fat component. Where a tumor andmature cystic teratoma coexisted, ascites and the invasion of adjacent structures were also common. CONCLUSION: If, in an older patient, CT scanning reveals an ovarian cystic tumor with a large solid portion but nocalcification or fat, coexistent tumor should be suspected.