The authors report a case of recurrent mammary leiomyosarcoma in a 50-year-old woman. The neoplasia, with a recognized clinical evolution of 11 years, was resected on two occasions and had not metastasized. Microscopic examination showed 4 mitoses/10 high-power fields, moderate cytologic atypia, and, ultrastructurally, abundant myofibrils with condensations. Immunoperoxidase stains had positive results for muscle-specific antigen and showed focal reactivity for epithelial membrane antigen and S-100 protein. Analysis of the ten cases (including the present one) reveals that this neoplasm has appeared with greater frequency in women with an average age of 52 years. All neoplasms have been limited to the breast at the time of diagnosis. As a group, they have better prognosis than other sarcomas of the breast, although the possibilities of recurrence or dissemination exist, even many years after the primary extirpation. The size of the tumor and mitotic activity seem to be of little prognostic value. Mammary leiomyosarcoma shares clinical and pathologic similarities with subcutaneous leiomyosarcoma in other anatomic sites.