The aims of this study were to define the initial pathological and clinical characteristics, and prognostic factors of patients with primary breast malignant lymphoma (PBL). All patients treated at the Institut Curie for lymphoma with breast involvement were reviewed. A pathological review of all cases was performed. Forty-five cases were selected in whom 38 cases were of diffuse large B-cell lymphoma. A complete analysis was then performed on these 38 patients. Twenty out of 28 cases (71%) of cases were Bcl-2 positive and four out of 28 (14%) had a CD10 positive staining. Peculiar initial characteristics showed nodal involvement in 58% of the cases and two or more extra-nodal sites in 31% of the cases. Among the 37 patients for whom all data were available, and according to the International Prognostic Index, 19 patients (51%) were classified in the low-risk group, 5 cases (14%) in the low- to intermediate-risk group, 6 patients (16%) in the intermediate- to high-risk group, and 7 (19%) case in the high-risk group. At the end of initial therapy, 34 patients (89%) achieved CR. With a median follow-up of 96 months, 18 patients (47%) relapsed of whom 3 had a relapse in central nervous system site. The 5-year disease-free (DFS) and overall survivals (OS) were 54% and 61%, respectively. In multivariate analysis, the presence of 2 or more extranodal sites was prognostic for lower DFS (P = 0.0008) and OS (P = 0.09), and a performance status > or = 1 was prognostic for lower OS (P = 0.005). Finally, when our series was compared with a historical series of 111 patients with aggressive nodal lymphomas, we observed significant lower survival rates in localized PBL (P < 0.03). Initial breast localization has a pejorative impact on the outcome of patients with Non-Hodgkin's Lymphoma (NHL), with an impressive adverse influence of additional extranodal sites. These results suggest a specific management of NHL with breast involvement.