A 63-year-old woman underwent breast-conserving resection and axillary lymph node dissection for bilateral breast cancer in December 2008. Histopathological diagnosis for the right breast cancer was t=1.3 cm, n=1/29, estrogen receptor (ER) (+), progesterone receptor (PgR) (+), human epidermal growth factor receptor 2(HER2) (-), and that for the left breast cancer was t=1.8 cm, n=9/28, ER (+), PgR (+), and HER2 (-). She was administered adjuvant chemotherapy (4 courses of fluorouracil, epirubicin, and cyclophosphamide [FEC 100] and 4 courses of docetaxel[DTX], 75 mg/m²), letrozole, and bilateral radiation therapy for the remaining breast tissue. She noticed a mass in the left breast in December 2013. Fine-needle aspiration cytology and core-needle biopsy indicated a malignant phyllodes tumor or stromal sarcoma. Positron emission tomography-computed tomography (PET-CT) revealed the accumulation of fluorodeoxyglucose (FDG) only in the mass. The tumor enlarged rapidly to more than 5 cm during the pre-operative period. In January 2014, the patient underwent left mastectomy. The histopathological diagnosis was malignant phyllodes tumor in the left breast, with a tumor diameter of 7 cm, and negative margins. Presently, 6 months after the operation, the patient is alive without recurrence.