Primary breast sarcoma, arising from connective tissue within the breast, is extremely rare, accounting for less than 1% of all primary breast malignancies and no more than 5% of all sarcomas. The rarity of this pathology limits most studies to case reports and small retrospective studies, which has led to a lack of consensus on the clinical management. We report a clinical case of a 52-year-old woman, perimenopausal, previously healthy, with regular breast surveillance, who presented with a large (>20 cm) and rapidly expanding hypervascularized tumor of the left breast developed over 10 days, with a very thin preulcerative skin over the last 4 days. There was no systemic dissemination. The patient was submitted to total mastectomy and excision of axillary adenopathy. The tumor was diagnosed histologically as malignant phyllodes tumor associated with areas of high-grade sarcoma. Due to rapid growth and aggressive histological characteristics, adjuvant chemotherapy and radiotherapy were performed. There is a lot of evidence that tumors larger than 5 cm are associated with a poor prognosis. Despite the poor prognosis associated with this aggressive entity, the patient had no recurrence during 5 years of follow-up. We review the relevant literature about primary breast sarcomas.