PURPOSE: To evaluate the sonographic and mammographic findings in Paget's disease of the breast and to assess the usefulness of sonography. MATERIALS and METHODS: Sonographic (n=13) and mammographic findings (n=12) of thirteen patients (age ranged from 25 to 86 years with the mean age of 56 years) with pathologically proven Paget's disease of the breast were retrospectively reviewed and compared. Clinical manifestations of the subjects were reviewed using patients' medical records. On sonography, any change in subareolar, the presence of mass, the configuration of the mass if present, the changes of the main lactiferous ducts and the presence of calcification were analyzed. On mammography, the subareolar density, parenchymal distortion, the presence of mass and the presence and type of calcifications were analyzed. RESULTS: The frequent clinical manifestations of Paget's disease were nipple retraction(7/13), bloody nipple discharge(7/13), nipple eczema(4/13) and subareolar mass(4/13). On sonography, mass was the most common(12/13) feature, and the mass was seen as an irregular hypoechoic mass. The irregularly dilated major lactiferous duct associated with a mass(3/13), calcification(2/13), subareolar low echogenicity(1/13) and parenchymal distortion(1/13) were also observed. The most frequent mammographic finding was increased subareolar density(9/12), followed by parenchymal distortion(8/12), microcalcification(6/12) and the presence of mass(3/12). The types of microcalcification seen on mammography were branching(4/6) and granular(2/6) shapes. Mass in the breast was more frequently detected on sonography(12/13) than on mammography(4/12). The irregularly dilated major lactiferous duct(3/12) associated with the mass could not be demonstrated on mammography. On the other hand, breast sonography was inferior to mam ography in the detection rate of microcalcification. CONCLUSION: In diagnosis of Paget's disease in the breast, sonography was more useful than mammography in the detection of a mass and in the evaluation of the main lactiferous ducts.