To compare the specific information of each imaging modality of US (7.5MHz, water path system) and galactography, and to appreciate clinical applicability of US as a primary diagnostic method in breast tumor causing nipple discharge, we performed a comparative study of each method in 15 cases of the ductal tumor of breast. To evaluate the demonstrable smallest anatomic unit of the lactiferous system by US, 30 normal lactating breasts were examined prospectively. The visible smallest lactating unit in the normal lactating breast by US is terminal duct lobular unit(TDLU). US is superior to galactography for detection of the mass less than 0.5cm in size with proximal ductal dilatation, and for evaluation of the multiple ductal lesions, in contrast, inferior to galactography for systemic visualization of involving ducts and for the detection of microcalcification. Sonographic diagnosis is limited in detection of small retromammillary masses, peripheral small masses without ductal dilatation and masses in excessively fatty breast. In conclusion US can be a primary diagnostic tool in suspicious breast tumor causing nipple discharge as a non-invasive method but galactography should be combined in the indicated cases.