Malignant phyllodes tumors of the breast are uncommon and complex to treat. This case involves a 39-year-old woman with a rapidly growing mass in her right breast measuring 32cm. The patient underwent a bilateral mastectomy with a right sentinel node biopsy and right chest wall reconstruction. The final pathology of the tumor revealed a malignant phyllodes tumor, with one of two right axillary lymph nodes positive for metastatic phyllodes tumor. Malignant phyllodes tumors should be taken into consideration in any rapidly growing breast mass. Further studies analyzing the treatment of malignant phyllodes tumors are necessary to reduce the risk of tumor recurrence and metastasis.