Primary lymphoma of the male breast is extremely rare. We report a case of a diffuse large B-cell lymphoma in a male patient. A 67-year-old man presented with a palpable mass in the right breast and ipsilateral axillary lymphadenopathy. At operation a 6 x 5 x 4-cm mass was excised, and a frozen section demonstrated malignancy. A modified radical mastectomy was then performed, together with axillary lymph node clearance. Histological examination established the diagnosis of a primary non-Hodgkin's lymphoma of the breast. The patient was referred for chemotherapy and died a year later from systemic disease involving the adrenals. The importance of early diagnosis is emphasized; this should be based on an excisional biopsy or aspiration cytology. As patients with primary breast lymphoma (PBL) have a better prognosis than those with carcinoma of the breast or patients with extranodal lymphomas, a multidisciplinary approach including surgery, radiotherapy, and chemotherapy when needed would result in a more favorable outcome.