PURPOSE: To determine which sonographic findings usefully differentiate between benign and malignant papillary tumors. MATERIALS AND METHODS: We retrospectively rev i ewed the ultrasonographic findings of 42 surgically proven cases of papillary breast lesions [11 malignant lesions (7 inva s i ve papillary carcinomas, 4 intraductal papillary carcinomas) and 31 benign intraductal pa-pillomas]. All 42 cases were classified sonographically as cystic or ductal, or solid type, and the shape, wall change, margin, internal echo-pattern, posterior echo change and other associated findings for the two types were then analysed. RESULTS: Among the 25 cases (5 malignant and 20 benign) of cystic or ductal type, tubular shaped lesions were more frequently benign (60%). In all 20 benign lesions the wall of cystic portion was well-defined, smooth and thin. The solid portion of the cystic type showed an illdefined irregular margin in four malignant lesions (80%) and a smooth margin in 19 which were benign (95%). The internal echo-pattern was heterogeneous mixed-echo in three cases of malignancy, and homogeneously hypoechoic in 19 benign lesions (95%). Posterior enhancement was seen in two malignant lesions (40%), while in 19 benign lesions (95%), there was no posterior echo change. There were 17 solid type lesions (6 malignant cases, 11 benign cases), and most of these, whether benign or malignant, were smooth, oval or lobulated, hypoechoic masses. Posterior enhancement, howeve r, was more frequently observed in malignant lesions (three cases, 50%) than in those which were benign (one case, 9%). CONCLUSION: In cystic or ductal type lesions, an ill-defined irregular thick cystic wall, an illdefined irregular margin, a heterogeneous mixed internal echo-pattern and posterior enhancement of the solid portion suggested malignancy. In solid type lesions, posterior enhancement was more frequently found in malignant than in benign lesions.