PURPOSE: To compare the radiologic findings of pyogenic abscess(PA) and inflammatory cancer(IC) of the breast, as seen on mammograms, US, and MR images and to evaluate the usefulness of the differential diagnostic findings of MRI for differentiation of PA and IC of the breast. MATERIALS AND METHODS: We retrospectively analyzed the mammographic, US and MR findings of nine histopathologically proven cases of PA and four cases of IC of the breast. Parenchymal density, mass density, skin thickening and calcification were evaluated by mammography, and the extent of lesion and its morphologic characteristics and changes of dermal lymphatics by US and MRI. The latter was also used to analyse signal intensity and enhancement pattern. RESULTS: Mammographic findings for both lesions were nonspecific. US showed that the pyogenic abscess was a hypoechoic and anechoic complex lesion with posterior acoustic enhancement, while inflammatory cancer was a spiculated lesion with a heterogeneous hypoechoic echotexture. On 3D-GRE dynamic enhanced M-RI, PA was shown in six cases(66.7 %) to be a lesion with superficial periareolar involvement, in contrast to the deep parenchymal spread seen in all cases of IC. A central non- enhanced portion with an irregular thick peripheral enhanced rim was seen in eight cases of PA(88.9 %) , and in all cases of IC an inhomogeneously enhanced spiculated lesion in parenchyma and a diffusely enhanced dermal and subcutaneous layer was apparent. An MRI time-intensity curve showed that the enhancement pattern was slow in five cases of PA(55.6 % ) and irregular in four(44.4 %), while for IC it was rapid in three cases(33.3 %) and irregular in one(11.1 % ). CONCLUSION: As compared with mammography and US, 3D-GRE dynamic MRI was a useful method for the differential diagnosis of PA and IC of the breast. The characteristic MR findings of PA were a central non-enhanced portion with an irregularly thick peripheral enhanced rim, located mainly in the superficial periareloar area and spreading into the parenchymal layer. An inhomogeneously enhanced spiculated solid mass in the parenchyma associated with diffuse dermal thickening and lymphatic dilatation was more suggestive of IC.