Fourteen cases of primary malignant lymphomas of the breast were found in the pathology files of the M. D. Anderson Hospital and Tumor Institute from 1944 to 1975. The lymphomas represented only 0.12% of 11,277 primary malignant breast tumors seen during the same period. There were no definite clinical features to distinguish the patients with lymphoma from those with breast carcinoma. All of the lymphomas had a diffuse pattern. Eight cases were classified as undifferentiated lymphoma, five as histiocytic, and one as poorly differentiated lymphocytic, convoluted cell type. Four patients had mastectomies and the remainder biopsies as their sole surgical procedure. Eight patients received post-surgical radiotherapy and all eventually had chemotherapy. The five-year survival rate for the 13 patients with follow-up was 49%. Patients with histiocytic lymphoma appeared to have a more favorable prognosis than those with the undifferentiated type. Six of the latter patients are dead with a median survival of seven months, comparable to the reported survival of patients with American Burkitt's lymphoma. The patient with the convoluted cell type has developed acute blastic leukemia and is currently under therapy.