Ovarian serous borderline tumors (SBTs) are characteristically associated with an indolent course. Recurrences are often delayed and usually show morphologic features of SBT or low-grade serous carcinoma. Transformation to high-grade carcinoma has rarely been documented. We report 2 cases of ovarian SBTs that recurred early as high-grade carcinomas. The first was a 50-year-old woman treated surgically and with chemotherapy for a FIGO stage 1C SBT with microinvasion, who experienced a recurrence in an axillary lymph node at 27 months. The recurrent tumor consisted of well-differentiated papillary serous tumor that resembled the primary tumor and poorly differentiated serous carcinoma. The patient died of progressive disease 43 months after her initial presentation. The second case was a 61-year-old woman treated surgically and with chemotherapy for a stage 3C micropapillary SBT with noninvasive implants. Eighteen months later, an incisional hernia was found that contained high-grade sarcomatoid-type carcinoma with microscopic foci of better differentiated tumor that resembled the primary SBT. This patient is alive with disease 24 months after her initial presentation. Whereas the malignant potential of SBTs remains controversial, the cases described herein demonstrate that SBTs can behave unpredictably and may rarely transform into high-grade carcinoma.