To evaluate the predictive ability of sonographic tumor characteristics to differentiate benign from malignant tumors, we examined 3093 breast tumors (2360 benign and 733 malignant tumors) with ultrasonography. The ratio of the longest dimension to the anteroposterior diameter of benign tumors was significantly larger than that of malignant tumors (1.88+/-0.1 versus 1.69+/-0.02, P < 0.0001). Shape, margins, echogenicity, internal echo pattern, retrotumor acoustic shadowing, compressibility, and microcalcification were significant factors in the logistic regression model. The sensitivity, specificity, positive predictive value, negative predictive value, and accuracy of breast sonography for malignancy were 86.1, 66.1, 44.1, 93.9, and 70.8%, respectively. Biopsy of the tumor for pathologic diagnosis is recommended if sonographic features are suggestive of malignancy.