Lymphoproliferative disorders, manifesting either as a primary lesion or as part of a generalized process, rarely involve the breast. The primary malignant lymphoma of the breast is rare. The cytologic features of this neoplasm have not been reported in Korea thus far. We described a case of primary malignant lymphoma of the breast, as diagnosed by fine needle aspiration cytology (FNAC) in an 84-year-old female. FNAC smears showed high cellularity. The main cytologic feature observed was a carpeting of monotonous, atypical, enlarged lymphoid cells. These lymphocytes showed relatively round elliptical nuclei with some indentation. One or more prominent nucleoli were seen. Biopy from the breast mass revealed diagnostic features of non-Hodgkin's lymphoma, diffuse large B-cell type.