The breast tumor cystosarcoma phyllodes probably does not Rpresent an entity, but rather a spectrum of disease including a benign form, a histologically malignant form, and a metastatic form. In an attempt to better understand the metastatic variant, all 66 cases which could be found in the english literature and an additional case from this institution were reviewed.The tumor often arises from a fibroadenoma, but its etiology is uncertain. Factors that determine the benignity or malignancy of the tumor are unknown, but a lower fertility rate in patients with metastases warrants interest.Incidence is low; cystosarcoma makes up about 0.5% of alI breast tumors and only 3-12% of these are metastatic.The tumor presents as a breast mass, often of large size, but there is no clinical feature peculiar to the metastatic variant.Gross and microscopic pathologic findings cannot accurately forecast the metastatic potential of the tumor, but are of more predictive value than the clinical presentation. The clinical course is the only sure means of determining metastatic potential at present.