- A series of 27 cystosarcoma phyllodes diagnosed in 23 patients is presented. Treatment consisted of wide local excision with a rim of normal tissue for the histologically benign-appearing tumors, and simple mastectomy for the histologically malignant-appearing tumors. Of the three tumors with a malignant appearance, one metastasized distantly and the patient died; the other two patients were apparently cured. Of the 24 benign-appearing tumors, 3 recurred locally and were cured by reexcision. None behaved in a malignant fashion. There was one case of bilateral cystosarcoma phyllodes, and one tumor was associated with carcinoma of the breast. The local recurrence rate of 11% (3 out of 27) and mortality rate of 4% (1 out of 23) appears to justify this therapeutic approach. The precise indications for mastectomy in this disease are still unclear.