We have reviewed the histopathological, clinical outcome and immunohistochemical status in 21 women with cystosarcoma phyllodes (CSP) tumors of the breast. We assessed 12 tumors as histopathologically benign and 9 tumors as malignant. The median patient ages in benign and malignant CSP tumors were 39.6 and 45.4 years of age, respectively. The stromal cellularity, stromal cellular atypism, high mitotic activity, atypic mitoses, stromal overgrowth, infiltrative tumor contour, and heterologous stromal elements were significant features of the malignant CSP tumors. Benign CSP tumors were predominantly of fibroadenomatous architecture with cellular stroma (mild or moderate) and some distortion and elongation of glandular elements. Five malignant CSP tumors were stained positively with p53, and 6 malignant CSP tumors were stained immunohistochemically with Ki-67. All benign CSP tumors were negatively stained for p53 and Ki-67. The patients with benign CSP tumors were treated with local excision ( n=11) and with subcutaneous mastectomy ( n=1). Malignant CSP tumors were treated with wide local excision ( n=1), partial mastectomy ( n=1), simple mastectomy ( n=2), and modified radical mastectomy ( n=5). Two patients with a high mitotic rate and high values of p53 and Ki-67 received additional radiotherapy and chemotherapy. One case had liver metastasis. This tumor had high mitotic figures, stromal overgrowth, severe stromal cellularity, and 20% Ki-67 and mild p53 positivity. We suggest that p53 and Ki-67 can play an important role in predicting prognosis and yielding additional therapy besides conventional prognostic factors in the treatment of the CSP patients.