Objective: extramedullary plasmacytomas of the breast are extremely rare, especially those that are not associated with multiple myeloma and they can mimic mammary carcinoma. Method: case report:69-year-old woman presented a palpable mass in the left breast previously diagnosed as a lobular carcinoma by core biopsy performed in other service. In our institution a conservative surgery with lymph node dissection was proposed. Results: Frozen section was required and the gross examination showed a 4.0×4.0×3.0 cm firm-elastic nodule with surgical margins free of tumor and the microscopic examination showed tumor cells with abundant cytoplasm with hyaline appearance. The paraffin sections showed a proliferation of plasma cells with moderate atypia. The immunohistochemical study confirmed the diagnosis of plasmacytoma. There was no axillary lymph node involvement. There were no bone lesions in the additional investigation. Conclusion: primary plasmacytomas of the breast are extremely rare and have to be included in the differential diagnosis with breast carcinomas especially in material of core biopsy.