A 34-year-old woman visited our hospital complaining of a rapid enlargement of masses at the right breast and right axilla, Biopsy of the breast revealed malignant lymphoma, and Br+Ax+Mj+Mn was performed. Histologically the tumor was a diffuse lymphoma in LSG classification, mixed type malignant lymphoma, and Pre-B-originated one. Postoperative VEPA and radiation therapies resulted in a transient remission, however, after 7 months she experienced a recurrence in the peritoneum, resisted various kinds of therapies, and died of general metastasis after 11 months. It might be diagnosed as systemic malignant lymphoma forming a mass in the breast. Malignant lymphoma originating in the mammary duct is relatively rare, and 121 cases have been reported in Japan. The avaraged age (47.7) when the patients first had the disease was slightly younger than that of brast cancer patients. The masses as many as about 1/4 of them were found in the right side C resion, 1/4 of the patients were accompanied by a rapid enlargement, and about a half of them had tumors with the maximum diameter of 5 cm or more. Operation + chemotherapy regimen was the most common. It has poorer prognosis than that of breast cancer. Moreover, a change in LDH appers to reflect the stage of the disease. Eleven cases originated in B cell alone have been reported up to now.