PURPOSE: To evaluate the morphologic differential diagnosis of benign and malignant ductal breast tumors, as seen on US. MATERIALS AND METHODS: US findings in 29 pathologically proven cases of ductal breast tumor were retrospectively reviewed. All patients were female and their mean age was 42 years. Nineteen tumors were benign and ten were malignant, and all ductal or cystic lesions showed solid masses. According to the location of the mural nodule, we classified the sonographic appearance of these tumors into three types : intraductal, intracystic and amorphic. The intraductal type was divided into three subtypes: incompletely obstructive, completely obstructive and multiple mural nodules. For the intracystic type, too, three subtypes were designated : the intracystic mural nodule (mural cyst), intracystic mural nodule with the duct (mural cyst+duct) and intracystic multiple mural nodules. The amorphic type is defined as an atypical ductal tumor with the mural nodule extending into adjacent parenchyma. RESULTS: The margin of the duct or cyst was smooth in 68.4% of benign, and irregular in 90% of malignant ductal tumors. Internal echogeneity of the duct or cyst usually showed homogeneity in both benign and malignant tumors. 73.7% of tumors connecting the duct were benign and 50% were malignant. In benign tumors, 52.6% of mural nodule had an irregular margin, while in malignant tumors, the corresponding proportion was 100% ; both types usually showed heterogeneous hypoechogeneity. Among benign tumors, the most common morphologic type was the intraductal incompletely obstructive subtype (36.8%) ; among those that were malignant, the amorphic type was most common, accounting for 40% of tumors. No amorphic type was benign and no incompletely obstructive subtype was malignant. CONCLUSION: When ductal breast tumors are morphologically classified on the basis of sonographic findings, the intraductal incompletely obstructive subtype suggests benignancy, and the amorphic type, malignancy. The morphologic classification of ductal breast tumors based on sonography is therefore useful for the differential diagnosis of benignancy and malignancy.