Two cases of primary malignant lymphoma of the breast were found, with breast cancer as the provisional clinical diagnosis for these two cases. Estrogen receptor determination via monoclonal antibody technology was performed in one case and the result was negative. Frozen section examination of another case during operation revealed chronic inflammation. The definite diagnosis of these two cases was finally obtained by paraffin section histological examination. Primary malignant lymphoma of the breast, although rare, should be included in the differential diagnoses of a breast lump in order to achieve earlier diagnosis and to thus plan an optimal therapy.