The surgical treatment and outcome of 31 women with a pathologic diagnosis of cystosarcoma phyllodes at Roswell Park Memorial Institute were reviewed. Surgical excision was performed upon 20 patients with a mean tumor size of 4.5 centimeters (range of 1.0 to 9.0 centimeters). Mastectomy was performed initially upon 11 patients with a mean tumor size of 15 centimeters (range of 3 to 40 centimeters). Local recurrence was seen in five patients treated by excision and one patient treated by mastectomy. All patients retreated by repeat excisions were controlled; one patient retreated with mastectomy died with local recurrence and distant disease. Routine axillary dissections were not performed. Involvement of the axillary lymph nodes occurred in one patient and was associated with advanced systemic disease. Local recurrences did not imply associated systemic spread and were controlled in five of six patients by repeat excision or mastectomy. Although the benign behavior of this tumor is commonly emphasized, four of the patients we studied died of malignant spread of cystosarcoma phyllodes.