Breast ductoscope is a fiberoptic endoscope used for examining the distal breast ducts under direct vision in order to identify the source of pathologic nipple discharge. The purpose of this study was to investigate the reliability of intra-operative breast ductoscopy in patients with pathologic nipple discharge, which could not be identified by radiologic tests. Between April 2002 and March 2007, breast ductoscopy was performed in 34 patients who had pathologic nipple discharge with no radiologic evidence about the source. The procedures were carried out under general anesthesia and ductoscopic findings were as well as the histopathology of the specimens were recorded and documented. In 88%, (30 of 34) of the patients, endoscope was successfully introduced into the external orifice of the ducts at the nipple and proximal breast ducts were successfully visualized. Ductoscopy revealed intraductal lesions (i.e., ductal obstruction, intraductal papilloma, red patches, and erythematoid platter) in 20 patients (66%). Among the 20 patients with visible endoluminal pathology, nine had a papilloma and eight had signs of either acute inflammation (bleeding, erythema) or previous inflammation with healing (adhesions and blocked ducts). In two cases, invasive breast carcinoma was identified, one of which was ductal carcinoma in situ (DCIS) with minimal invasion. In both cases, there had been blocked ducts. In one case DCIS was identified. Breast ductoscopy is a reliable and easy-to-use method to demonstrate the source of pathologic nipple discharge in cases with bleeding and other intraductal lesions.