Primary breast lymphoma. An immunohistologic study of 20 new cases. Primary malignant lymphomas of the breast (PBL) are uncommon. The authors report the clinical, histologic, and immunoperoxidase findings on 20 cases recorded at the Alberta Cancer Registry over the last 23 years. These cases were then added to material on 257 cases abstracted from the literature and analyzed. It was found that there are two clinicopathologic types of PBL. The first affects pregnant or lactating women with bilateral, diffuse disease, is rapidly fatal, and corresponds histologically to a Burkitt's-type lymphoma. The second is unilateral at presentation and afflicts a broad age range, but primarily older women. This has a variable course only part of which is predicted by histologic grade and stage. Tumor size, treatment, and side of presentation were not found to be significant prognostic factors. Histologically, these tumors can be grouped into large cell B-cell lymphomas, monocytoid B-cell lymphomas (MBCL), and undifferentiated, some of which may be T-cell. Evidence suggesting that the MBCL of breast are the equivalent of the malignant lymphomas of the mucosa-associated lymphoid tissues (MALT) is reviewed. The breast is a hormone-dependent member of the MALT and therefore it is interesting that two of these tumors were strongly positive for estrogen receptors. 