BACKGROUND: Primary non-Hodgkin's lymphoma of the breast (PBNHL) is a rare neoplasm and PBNHL occuring in a man is extremely rare. METHODS: We report herein two cases of PBNHL and discuss methods of diagnosis and treatment. RESULTS: The first patient was a 35-year-old woman who presented with an elastic-hard mass in her left breast and adenopathy in her left axillary fossa. Mammography and ultrasonography suggested left breast cancer. Frozen sections demonstrated PBNHL histologically. Quadrantectomy of the breast was performed followed by chemotherapy. The second patient was a 65-year-old man with an elastic-hard mass in his left breast and left axillary lymphadenopathy. Mammography and ultrasonography suggested breast cancer or a soft tissue tumor. Intraoperative histological examination revealed a diagnosis of non-Hodgkin's lymphoma (NHL). A simple mastectomy and postoperative chemotherapy were performed and the patient had a good prognosis. CONCLUSION: A multidisciplinary approach including surgery, chemotherapy, andradiotherapy is indispensable in treating PBNHL, after diagnosis by excisional biopsy or aspiration cytology.