PURPOSE: To analyze and correlate mammographic/ sonographic features with pathologic findings in patientswith infiltrative ductal carcinoma with the mammographic 'halo sign'. MATERIALS AND METHODS: We retrospectivelyreviewed 27 cases of surgically confirmed infiltrative ductal breast carcinoma with the mammographic 'halo sign',analysing the mammographic/sonographic findings, clinical records and pathologic findings. RESULTS: Twenty-sevenpatients with infiltrating ductal breast carcinoma with the mammographic 'halo sign' were aged 32-72 (mean 49+/-12)years. For periods of between 2 days and 5 months(mean, 1.9 months), all had complained of a palpable mass.Histologic sections indicated parenchymal compression(27/27), microscopic infiltration of tumor margin(11/27) andparenchymal fat compression(15/27), without a true capsule. Infiltrating ductal carcinomas with the mammographic'halo sign' were nuclear grade 1 (17/27) or histologic grade III (14/27). Mammography (complete halo:3;partialhalo:24) showed a round(15/27) or macrolobulated (9/27) mass (size : 2.5+/-1cm) which showed partialspiculation(17/27) or-rarely-calcification(2/27). Sonography revealed a hypoechoic mass with a thinboundary(13/15);in five of these 13 cases, pathologic examination revealed microscopic infiltration of the tumormargin. Bilateral shadowing (8/15), posterior enhancement(13/15), and heterogeneous internal echogenicity(13/15)were also noted on sonography. CONCLUSION: When correlated with the histopathologic findings, infiltrating ductalbreast carcinoma with the 'halo sign' showed a relatively poor pathologic grade, which is known to be one of theprognostic factors. Surrounding parenchyma and fat compression at the margin of a tumor are considered to be oneof the causes of the mammographic 'halo sign'.