An analysis of forty cases of cystosarcoma phylloides of the breast was undertaken to clarify the relationship between histology, surgical treatment and prognosis of this fibroepithelial tumor. Thirty-eight female patients 12 to 85 years of age, treated for this diagnosis over a 17-year period, were studied retrospectively. Histologically malignant lesions were diagnosed in 17 cases: six were treated by local excision, one by subcutaneous mastectomy with prosthetic implant, four by simple mastectomy, three by modified, two by radical mastectomy, and one by biopsy only. Of the 23 histologically benign tumors, 18 were treated by local excision, three had simple mastectomy, and one had subcutaneous mastectomy with prosthetic implant. Three recurrences, observed (7.5%) among patients with benign tumors, were locally excised. Wide local reexcision has controlled the disease to date after average follow-up of 32 months. In the malignant group, metastases developed in four patients (10%). Three of these patients are dead of disease and one is being treated by chemotherapy. Malignant histology seemed to correlate directly to the presence of pain (46.6%), size of the tumor (average 7 cm in diameter), and older age (average 52 years), but there was no correlation with prognosis. Recent reports on the subject advocate wide local excision for small benign lesions and simple mastectomy for larger or malignant ones. Our data indicates that the histologic appearance does not correlate with the clinical behavior of this neoplasm, and the choice of the procedure does not alter the long-term result, provided that the tumor is completely excised. More radical procedures than wide local excision are not justified for cystosarcoma phylloides which behaves more like a soft tissue sarcoma than a breast gland tumor.