Clinical and pathologic findings in 19 cases of primary non-Hodgkin's lymphoma of the breast collected from several hospitals in Japan were reviewed. All patients were women (median age, 45 years) and they usually had breast masses that had recently become enlarged. The sites of the lesions were the right breast in eight cases, the left breast in eight, and both breasts in one. The locations of two masses were unknown. Lymphoma recurred in the opposite breast in three cases 14, 23, and 23 months after surgery. Histologically, diffuse large cell lymphoma was the most common form of disease (63%). One lesion was a follicular lymphoma. The so-called lymphoepithelial lesion, a characteristic finding for mucosa-associated lymphoid tissue type lymphomas, was observed in eight cases (42%). Immunohistochemical analysis revealed that all but two tumors were of B-cell type; such findings confirmed morphologically based conclusions. Histologic and immunohistochemical evidence of lymphocytic mastopathy, a recently described autoimmune disease of the breast, was found in most of the cases. Formation of lymphoid follicles in or around the tumors was found in five cases (26%). Based on these findings, it is suggested that most mammary lymphomas are B-cell tumors and they may be associated with coexisting or antecedent lymphocytic mastopathy.