Rhabdomyosarcoma of the breast is a rare tumor that is characterized by its rapid growth, large size, and poor prognosis. It most often presents as a pure neoplasm and infrequently as the stromal component of a cystosarcoma phyllodes. The clinical and pathological features of a cystosarcoma with a rhabdomyosarcomatous stroma occurring in a 45-year-old woman are presented, and the results are discussed in reference to the 24 published cases of mammary rhabdomyosarcoma. The patient was treated by radical mastectomy and died 2.5 years later with pulmonary and cerebral metastases. Though two long-term survivors have been reported, cures are infrequent and no form of therapy has been uniformly successful.