PURPOSE: The purpose of this study was to assess the mammographic features and pathologic outcome of category 4 lesions using the Breast Imaging Reporting and Data System(BI-RADS), and to evaluate the significance of final assessment categories. MATERIALS AND METHODS: Using BI-RADS, the interpretations of 8,134 mammograms acquired between January 1997 and May 1998 were categorized. From among 161 lesions categorized as "4"("suspicious abnormality") and pathologically confirmed by surgery or biopsy, we analysed 113, found in 66 patients. RESULTS: The pathologic outcome of these 113 lesions was as follows: infiltrating ductal carcinoma, 17.7%(20/113); DCIS(ductal carcinoma in sitv), 8.0 %(9/113); ADH(atypical ductal hyperplasia), 5.3 % (6/113); DEH(ductal epithelial hyperplasia), 1.8 %(2/113); ductectasia, 0.9 %(1/113), FCD(fibrocystic change), 27 .4 %(31/113); fibroadenoma, 7.1 %(8/113); stromal fibrosis, 9.7%(11/113); normal parenchyma, 7.1 % (8/113); other pathology, 15.0 %(17/113). The most frequent mammographic features of BI-RADS category 4 lesions were irregular mass shape(41.2 %), spiculated mass margin(52.3%), amorphous calcification(47.3%) and clustered calcification distribution(37.1% ). CONCLUSION: Because category 4 lesions account for about 25.7 % of all breast malignancies, mammographic lesions in this category ("suspicious abnormality")should be considered for supplementary study and breast biopsy rather than short-term follow-up. Initial pathologic findings can thus be confirmed.