Cystosarcoma phyllodes of the breast are rare, and it accounts for less than 1% of the malignant tumors of the breast. Its clinical and radiological characteristics are those of benign tumors, and only a fast tumoral growth can evoke diagnosis. The diagnosis is confirmed by anathomopathology and immunohistochemistry. Surgical extended resection is the treatment of choice. Adjuvant radiotherapy may be administered in patients with high-grade tumors, positive surgical margins, or postoperative recurrence. Here we present a case of a woman affected by a sarcoma phyllodes tumor, an uncommon oncologic disease.