Primary non-Hodgkin lymphoma of the breast is a rare disease. Primary mucosa-associated lymphoid tissue lymphoma is even rarer, and bilateral involvement is exceptional. We describe a case of primary bilateral breast mucosa-associated lymphoid tissue lymphoma with bilateral atypical ductal hyperplasia and bilateral localized amyloidosis in a 64-year-old woman with a history of arthritis and systemic lupus erythematosus and its clinical, histologic, and immunohistochemical features. Microscopic examination of the breast lesion showed dense periductal and perilobular small and plasmacytoid lymphocytes with eosinophilic amyloid in the vessels and the stroma. Bilateral single foci of atypical ductal hyperplasia were also noted. Fine needle aspiration showed small and large lymphocytes and plasma cells. Molecular analysis demonstrated a heavy chain immunoglobulin H gene rearrangement. Flow cytometry studies showed an abnormal B-cell population. The combined histologic, paraffin immunohistochemistry, flow cytometry, and molecular results were considered diagnostic for low-grade mucosa-associated lymphoid tissue lymphoma. The patient underwent bilateral local breast radiation without other organ or site involvement.