Malignant cystosarcoma phylloides is a rare breast cancer which frequently recurs locally before metastasizing. Local recurrence of cystosarcoma phylloides is unlike adenocarcinoma of the breast since it does not necessarily indicate systemic metastases; and, therefore, aggressive surgery can cure a number of patients with locally recurrent tumor. The present report is an extreme example of a patient with locally recurrent malignant cystosarcoma phylloides, and it illustrates the value of radical surgery in controlling this disease which had previously recurred 21 times. Fifteen months following sternal resection, the patient enjoys her longest tumor-free interval in 16 years.