Primary mammary lymphoma is rare, and little is known about the immunophenotype of such cases. The authors studied both primary and secondary breast lymphomas with a broad panel of T- and B-cell markers using paraffin-embedded tissue and the avidin-biotin immunoperoxidase method. Cases of primary B-cell lymphoma were further tested to determine light and heavy chain type. Thirty-five cases were analyzed, including 16 primary lymphomas. Diffuse large cell lymphoma was present in ten of 16 primary and 14 of 18 secondary cases. Lymphoepithelial lesions in ducts and lobules and frequent vascular involvement were found in both primary and secondary cases. Immunohistochemistry studies on 13 tumors revealed all of the primary tumors to be B-cell in origin, except for one case of primary T-cell lymphoma; to the authors' knowledge, this represents the first description of this entity. Fifteen of 17 secondary tumors exhibited B-cell markers and one of 17 exhibited T-cell markers; in only one case could lineage not be determined. Among primary B-cell cases, IgM was found to be the most frequent heavy chain type; IgA reactivity was found in only one case. Survival was related to stage and histologic characteristics; patients with Stage II disease and higher grade histologic lesions had a worse prognosis. Half of the patients with primary lymphoma have had recurrent disease. Although local recurrences were observed, the authors also saw a tendency for recurrence in other extranodal sites. The authors conclude that, although primary lymphoma of the breast is compatible with long-term survival, a significant number of patients eventually die of their disease. As with other extranodal lymphomas, survival rate appears most related to stage of disease. Also, the vast majority of primary tumors are B-cell in origin, express IgM heavy chain, and can be considered tumors of mucosa-associated lymphoid tissue.