PURPOSE: This study was designed to investigate differences in ultrasonographic findings between malignant and benign mammary duct ectasia. METHODS: From January 2003 to June 2005, 54 surgically proven mammary duct ectasia lesions depicted on sonograms were included in this study. We evaluated the ultrasonographic (US) findings in terms of involved ductal location, size, margin, intraductal echogenicity, presence of an intraductal nodule, calcification, ductal wall thickening and echo changes of the surrounding breast parenchyma. The US findings were correlated with the pathological features. RESULTS: Of the 54 lesions, 46 lesions were benign and eight lesions were malignant. Benign lesions included an inflammatory change (n=7), ductal epithelial hyperplasia (n=7), fibrocystic change (n=18), intraductal papilloma (n=11), atypical ductal hyperplasia (n=2) and sclerosing adenosis (n=1). Malignant lesions included ductal carcinoma in situ (DCIS) (n=6), infiltrating ductal carcinoma (n=1) and mucinous carcinoma (n=1). On US images, the peripheral ductal location, an ill-defined margin, ductal wall thickening and a hypoechoic change of the surrounding parenchyma were features significantly associated with malignant duct ectasia. CONCLUSION: For ill-defined peripheral duct ectasia with ductal wall thickening and surrounding hypoechogenicity as depicted on US, the possibility of malignancy should be considered and radiologists should not hesitate to recommend a prompt biopsy.