Breast involvement by immunolymphoproliferative disorders is rare. Primaryand secondary malignant lymphomas of the breast are much more common thanmultiple myeloma, of which only 16 cases have been described. We report twocases of patients with an established diagnosis of multiple myeloma in whombreast involvement appeared during the course of the disease. We underlinediagnostic difficulties caused by the lack of clinical and radiologicalfeatures which allow us to differentiate between breast carcinoma and breastinvolvement by lymphoproliferative disease. Only fine needle aspiration and/orexcisional biopsy can differentiate between immunolymphoproliferativedisorders and epithelial or mesenchymal tumors of the breast.