Phyllodes tumors of the breast are rare fibroepithelial neoplasms with variable malignant potential. Treatment is primarily surgical; the use of radiation therapy is controversial. Here, we present the case of a 29-year-old female with a histologically malignant phyllodes tumor of the breast initially treated with wide local excision. At 18 months following surgery the patient experienced a local recurrence, which displayed sarcomatous degeneration on pathological examination. The patient subsequently underwent a total mastectomy and adjuvant radiation therapy. We discuss the decision to employ radiation therapy in the management of the patient's tumor.