The charts of 35 women with primary malignant non-Hodgkin's lymphomas (NHL) of the breast were retrieved from the files of the Istituto Nazionale Tumori, Milan, over a 30-year period (1957 to 1986). These cases represented 0.1% of the more than 25,000 primary malignant tumors of the breast treated during the same period. The median age of these patients was 57 years (range, 28 to 81 years). In most cases, the clinical diagnosis was carcinoma. The tumors were either Stage IE(48%) or IIE(52%) at presentation, and only two patients had B symptoms. The right breast was involved in 17 patients, the left breast in 14, and both breasts in two. According to the updated Kiel classification and the Working Formulation (WF) for Clinical Usage, three cases were lymphoplasmacytoid (immunocytoma) NHL (WF, A); three, centroblastic-centrocytic, follicular NHL (WF, B); four, centroblastic-centrocytic, diffuse NHL (WF, F); 17 centroblastic NHL (WF, G); three immunoblastic NHL (WF, H); two B-lymphoblastic NHL (WF, I); and one, a Burkitt-like NHL (WF, J). Treatment consisted either of a combination of surgery, radiation therapy, and chemotherapy or radiation therapy and chemotherapy. The follow-up period for 32 patients ranged from 6 to 161 months (mean, 45 months); 17 patients died of their disease. The prognosis appeared to be related to the histologic type and stage of the disease. Median survival periods were 63, 52, 42, and 47 months for centroblastic-centrocytic follicular, centroblastic-centrocytic diffuse, centroblastic, and immunoblastic NHL, respectively. The overall 5-year survival rate was 43%; the 5-year survival rate and the probability of freedom from progression at 5 years were, respectively, 61% and 50% for Stage I and 27% and 26% for Stage II disease.