This study focused on unusual histologic features observed in 2 relapsed Brenner tumors. Case 1: A 67-year-old woman had a malignant Brenner tumor. The tumor relapsed was squamous-cell carcinoma. Case 2: A 51-year-old woman had a proliferating and malignant Brenner tumor with mucinous metaplasia. The relapsed tumor was a well-differentiated mucinous adenocarcinoma. These cases suggest that: (1) the relapsed tumor cells survived chemotherapy; and (2) the relapsed tumor cells transformed.