We report a case of insulin-dependent diabetic fibrous mastopathy, which clinically simulated breast cancer. The patient is a 55-year-old woman with stony-hard breast lumps, who had been treated with insulin for diabetes mellitus. Mammmography revealed a highly-dense mass shadow in the left breast and ultrasonography detected hypoechoic lesions with acoustic shadows in both breasts, suggestive of malignancy. Excisional biopsy was performed for the left breast lump. On histopathological examination, this was fibrosis with dense lymphocytic infiltration around lobules, which is a characteristic feature of diabetic fibrous mastopathy