The aim of this study was to describe the mammographic and sonographic findings of idiopathic granulomatous mastitis which is a rare inflammatory disease of the breast of unknown etiology. The clinical, radiologic, and pathologic features of 12 cases with idiopathic granulomatous mastitis were retrospectively reviewed. Mammography was performed in all cases, 8 of which showed a focal asymmetric density, 3 had a mass with irregular margins, and 1 had no abnormality. Sonography was performed in 10 cases, and a focal area with inhomogeneous and hypoechoic pattern was depicted in 6 cases, 4 of which were associated with internal tubular hypoechoic structures. One case revealed a hypoechoic mass consistent with malignancy. In 1 case sonography showed an edematous pattern involving nearly the entire breast. Two patients had normal sonograms. If a focal asymmetric density is seen in mammography and inhomogeneous hypoechogenity with internal hypoechoic tubular structures accompany ultrasonographically, these findings should suggest the possibility of idiopathic granulomatous mastitis; however, very often idiopathic granulomatous mastitis mimics a breast carcinoma clinically and the final diagnosis should be reached histopathologically due to high false-positive and false-negative mammographic appearances.