Phyllodes tumor is a rare, fast-growing, fibroepithelial neoplasm of the breast that can be classified as benign, borderline or malignant. The case reported here refers to a 25-year old patient with a lump on her left breast that grew rapidly in the postpartum period, reaching 21 cm in size. Core biopsy and immunohistochemical analysis indicated a diagnosis of high-grade phyllodes tumor. The patient underwent mastectomy of the left breast followed by breast reconstruction. Histopathology confirmed a malignant phyllodes tumor with chondrosarcomata’s and fibrosarcomatous differentiation and no angiolymphatic or perineural invasion. Phyllodes tumors are generally benign and differential diagnosis is usually made with fibroadenomas; however, clinical differentiation can be difficult in some cases. The definitive diagnosis is achieved by histopathology, which is able to differentiate benign from malignant forms of the disease. Standard treatment consists of local excision, with mastectomy being necessary in cases of borderline, large, malignant or recurrent tumors.