ABSTRACT
The borderline tumor is not benign but has low malignant potential, which accounts for 10-15% of all ovarian tumors. The mucinous borderline tumors make up approximately 40% of all borderline tumors. About 5% of mature cystic teratomas of ovary include some with mucinous cystadenoma. The malignant transformation is occurred in about 1-3% of patients who have a mature cystic teratoma. There has been reported frequently about the malignant transformation of a mature cystic teratoma. After the first description of a patient with mucinous borderline tumor associated with a mature cystic teratoma in 1988, the borderline tumorous change of a mature cystic teratoma has been reported rarely. This report presents a patient with atypical proliferating mucinous tumor arising in a mature cystic teratoma.