Ovarian borderline tumors usually occur in young patients and are associated with good clinical outcomes. A proportion of these tumors, particularly those with micropapillary features, may progress to low-grade serous carcinomas. Evolution of low-grade serous neoplasms to high-grade carcinomas is rare and infrequently reported. In this case report, we describe 3 cases of serous borderline tumor that progressed to high-grade carcinoma. Patient ages were 22, 35, and 47 yr. Two patients were initially diagnosed with ovarian serous borderline tumor, and recurred in the pelvis 3 and 10 yr later with sarcomatoid carcinoma. The third patient presented with low-grade serous carcinoma arising in a micropapillary serous borderline tumor, and recurred with a carcinosarcoma 3 yr later. All 3 patients had an aggressive clinical course: 2 died of disease and 1 patient has disease progression on chemotherapy. Occasional low-grade serous tumors of the ovary may transform to high-grade carcinomas, which can occur many years after initial diagnosis. The presence of a high-grade component significantly worsens patient outcomes.