Malignant alteration in cystosarcoma phyllodes is uncommon and almost always confined to the stromal component. A rare case of recurring cystosarcoma is reported. In the first recurrence, lobular carcinoma in situ was present within the tumor and the second recurrence revealed tubular carcinoma within the cystosarcoma. Epithelial proliferation of various sorts is not uncommon in cystosarcomas but only two previous instances of infiltrating carcinoma have been observed within these tumors. The problem of mammary carcinoma coexisting with cystosarcoma is reviewed and comparisons are made with a closely related neoplasm, the fibroadenoma.