Primary malignant lymphoma of the breast is rare, and Burkitt's lymphoma is especially rare. We report the case of a 44-year-old woman in whom Burkitt's lymphoma involving both breasts was diagnosed. The patient was referred to our hospital because of a diffuse, firm swelling, like a bulky ball, in both breasts. Fine-needle aspiration cytology (FNAC) of both breast masses revealed malignant lymphoma(ML), and diffuse large B-cell lymphoma was diagnosed based on the results of immunohistochemical studies of a core needle biopsy specimen. The gallium scan revealed very hot lesions in both breasts, but there was no evidence of disseminated disease. We instituted initial therapy, with cyclophosphamide, doxorubicin, vincristine, and prednisone (CHOP) plus intrathecal chemotherapy without performing a chromosome analysis, because the clinical course was very aggressive. A CR was achieved, but after eight cycles of CHOP therapy, the ML relapsed. We then resected the mass in the left breast, and when examination of the surgical specimen revealed relapse of ML and t (8;11) (q24;q32) translocation, Burkitt's lymphoma was diagnosed. High-dose chemotherapy followed by peripheral-blood stem cell transplantation was performed, but the patient died 10 months after her initial presentation at our hospital.