A retrospective clinicopathologic evaluation of 42 patients with cystosarcoma phyllodes was undertaken to determine if tumor size, contour, degree of stromal atypia and mitotic activity were reliable indicators of clinical behavior. Excluding size, the latter three determinants showed a positive correlation with prognosis and served as the basis of a classification in which 18 benign, 5 borderline and 19 malignant cystosarcomas were diagnosed. The tumors occurred in women averaging 44.3 years of age who most often presented with a palpable occasionally painful mass with a median diameter of 5 cm. Excision or simple mastectomy were the more frequent forms of therapy. Local recurrences were experienced by 6 patients and occurred in all 3 categories of tumor. Only malignant neoplasms developed systemic metastases which was observed in 4 patients 3 of which have died. On borderline tumor recurred 14 times and eventually proved fatal as a result of contiguous pulmonary involvement. A plea is made to label the stroma of malignant cystosarcomas as to the cell(s) of origin so future investigators may evaluate the effect of various soft tissue patterns on prognosis.