We studied the morphologic, immunologic, and clinical features of 31 cases of malignant lymphoma involving the breast. Primary breast lymphoma occurred in nine women with a median age of 69 years (range, 51-87 years); median follow-up was 31 months (range, 9-67 months). Eight cases were low grade, one was high grade, and all expressed B-lineage antigens. Four cases had features of lymphoma of mucosa-associated lymphoid tissue (MALT); three were free of disease after excision alone at 10, 12, and 48 months, whereas the fourth relapsed with transition to immunoblastic lymphoma and died at 25 months. Four patients had follicular lymphomas, three of which relapsed, causing death from active disease at a median of 55 months (range, 25-67 months). One case of small noncleaved cell lymphoma relapsed, causing death at 31 months. Lymphoma secondarily involved the breast in 22 patients (21 women, one man) with a median age of 60 years (range, 39-83 years) at breast relapse; these patients were followed for a median of 88 months (range, 2-271 months) from primary diagnosis and 4 months (range, 0-116 months) from breast relapse. Nineteen patients had prior documented lymphomas (10 nodal or splenic, nine extranodal), and breast involvement most commonly occurred as part of widespread, predominantly nodal disease. Three patients had breast involvement by lymphomas that were generalized at diagnosis or staging. Thirteen cases were low grade (nine follicular), seven intermediate grade, and one high grade; 19 of 20 cases expressed B-lineage antigens, and one expressed T-lineage antigens. Four cases had features of MALT-type lymphoma; in these patients, isolated breast relapses were interspersed with other extranodal relapses, with interim resolution of disease after local or systemic therapy; two were free of disease and two were alive with localized disease on treatment at median follow-up of 60 months (range, 9-91 months). In contrast, 15 of 18 non- MALT lymphomas had widespread disease at breast relapse (median, 29 months; range, 0-259 months); 16 of 18 received systemic therapy, 10 died with active disease, and five of eight had disseminated active disease at last follow-up Primary breast lymphomas were commonly low grade. The follicular lymphomas had clinical behavior similar to nodal follicular lymphoma. Primary MALT-type lymphomas were a distinct subset with a potential for diseasefree survival after local therapy. Secondary breast lymphomas were heterogeneous and more commonly higher grade, although follicular lymphoma was the most common subtype. Non-MALT secondary breast lymphoma occurred in a setting of widespread disease in most cases. However, MALT-type lymphomas occurred as isolated relapses from other extranodal sites, with a potential for disease free survival after local therapy