We report a case of insulin-dependent diabetic fibrous mastopathy with special reference to the findings of computed tomography (CT). The patient was a 27-year-old woman with a history of insulin-dependent diabetes mellitus from childhood who presented with a right breast tumor. Physical examination showed a stony-hard, ill-defined but freely movable mass under the nipple of the right breast without nipple discharge. Mammography revealed a high-density mass shadow without microcalcifications or spicular formation. Ultrasonographic examination revealed an irregularly-shaped hypoechoic lesion with marked posterior acoustical shadowing. Contrast-enhanced CT revealed poor early phase contrast enhancement and slight delayed phase heterogeneous enhancement. Since core needle biopsy revealed fibrocystic disease, the lesion was suspicious for diabetic mastopathy. Incisional biopsy of the right breast lump was performed. On histopathological examination, the lesion showed fibrosis with dense lymphocytic infiltration around the lobules. Diabetic fibrous mastopathy was diagnosed. Physicians should be aware of the association of long-standing diabetes mellitus with the development of fibrous mastopathy. CT is considered a useful tool to differentiate diabetic mastopathy from breast cancer.