A retrospective study of 33 patients with cystosarcoma phyllodes was done. Eight of these patients had metastases, and the clinical and histologic criteria predicting the development of metastases were examined. The most reliable predictor was the presence of stromal overgrowth; this appears to be necessary for metastasis to occur. Other useful indicators of clinical behavior were the degree of mitotic activity, nuclear pleomorphism, and infiltrating margins. Based on these data and a literature review, the authors suggest close follow-up of patients whose primary tumors contain areas of stromal overgrowth because, in all series combined, the risk of metastatic spread in such patients was 72% within 5 years. Among these high-risk patients, local recurrence is another indication that metastasis is likely.