Lymphocytic mastopathy is a benign breast disease characterized by dense fibrosis, lobular atrophy, and aggregates of lymphocytes in a periductal and perivascular distribution. The condition affects young to middle-aged women and frequently shows an association with diabetes mellitus or autoimmune disorders. Here, we report a case of the disease clinically and radiologically mimicking primary breast neoplasms. The patient was a 50-year-old woman without diabetes who presented with two firm lumps in her right breast. Breast imaging findings from mammography, ultrasonography, computed tomography (CT), and magnetic resonance imaging (MRI), respectively, revealed an abnormal appearance suspicious of malignancy. A core-needle biopsy specimen showed atypical accumulation of lymphoid cells, which was not easy to differentiate from primary breast lymphomas. Moreover, (18)fluorodeoxyglucose positron emission tomography (FDG-PET) examination detected abnormal uptake in the same lesions. Histological examination of a surgically obtained specimen showed characteristic appearance of lymphocytic mastopathy, which predominantly consisted of B-lymphocytes. In our case, it was difficult to distinguish this entity from breast cancer or low-grade B-cell lymphoma without surgical biopsy.