Carcinoma arising in a phyllodes tumour of the breast. A rare case of a carcinosarcoma arising in a phyllodes tumour of the breast is reported. The clinical and pathologic features of this extremely rare neoplasm are described. The patient, a 50-year-old woman, presented with an invasive ductal carcinoma (T2N0M0) which was histologically identical to the underlying phyllodes tumour. Phyllodes tumours are rare, benign neoplasms with unpredictable biologic behavior. Carcinoma occurring in a phyllodes tumour is a particularly rare variant of carcinosarcoma. Phyllodes tumours should be treated by wide local excision, because of the difficulty of determining the exact amount of cancer present intraoperatively. If surgery is used, surgical resection margins should be assessed by histologic examination of the excised specimen. The prognosis of the phyllodes tumour is related to its histopathological grade. Invasive carcinoma in a phyllodes tumour indicates poor prognosis. Phyllodes tumours should be treated with wide excision, but the prognosis is difficult to assess precisely in the case of a malignancy arising in a phyllodes tumour. Wide local excision with histological examination of the surgical margins is a standard approach. Phyllodes tumours with both the phyllodes tumour component and the invasive carcinoma should be treated as malignant.