Primary non-Hodgkin's lymphoma of the breast is a rare neoplasm for which survival data vary among the reported studies. Thirty-one cases of diffuse large B-cell lymphoma of the breast, which had been seen in consultation from 1973 to 1985 at our institution, were reviewed. This represents the largest number of lymphomas of this histologic subtype reported to date in the English literature. Histologic examination and immunophenotypic analysis were performed and the results were correlated with clinicopathologic data. The patient population consisted of 29 females and 2 males with a mean patient age of 58.2 years. At the time of diagnosis, 26 patients had unilateral involvement (16 left, 10 right), and 5 had bilateral disease. Mean tumor size was 3.8 cm. Histologically, all cases showed a diffuse large B-cell lymphoma as classified by the Revised European-American Classification of Lymphoid Neoplasms (R.E.A.L Classification). Immunophenotypic studies on paraffin sections confirmed a B-cell lineage in every case. The majority of patients received chemotherapy and/or radiation therapy. The median survival was 36 months, confirming that this neoplasm has a poor prognosis.