ObjectiveTo investigate the methods and significances of the breast ultrasonographic breast imaging reporting and data system (BI-RADS) category 4 lesions divided into category 4a, 4b, and 4c, and to assess the risk of malig-nancy of lesions with BI-RADS category 4 in order to improve the accuracy of diagnosis. MethodsTwo hundred and eighty-eight breast lesions with BI-RADS category 4 confirmed by histopathology were collected. The ultrasonographic characteristics of benign and malignant lesions, containing the shape, aspect ration, margin, calcification, changes of the surrounding tissue, boundary, blood flow characteristics, internal echo, rear echo of the lesions, were comparatively anal-yzed, and the lesions with BI-RADS-US category 4 were divided into 4a, 4b, 4c according to these ultrasonographic charac-teristics and analyzed by statistics. ResultsThere were 192 malignant lesions and 96 benign lesions in the 288 breast lesions. There were statistical significances in the benign and malignant lesions with the shape, aspect ratio, margin, calci-fication, change of surrounding tissue, and boundary (P < 0.05), in other words, the proportion of these ultrasonographic characteristics were higher in the malignant lesions as compared with the benign lesions. But there were no significant differences of internal echo, rear echo, and blood flow characteristics between two lesions (P > 0.05). The positive predictive value of malignant tumor with BI-RADS category 4a, 4b, and 4c were 21.74%, 58.90%, and 91.78%, respectively, and there was significant difference (χ2=106.09, P=0.000). ConclusionsThe classification of breast lesions with BI-RADS category 4 is refined, it could more accurately assess the risk of benign and malignant breast masses. At the same time, it has an important clinical significance for diagnosis and treatment of benign and malignant breast masses.