We report cases of breast cancer with extensive intraductal component (EIC), the extent of which could be detected only by contrast-enhanced computed tomography (CT) scan. One case was a 32-year-old woman who presented left nipple bloody discharge without breast mass palpation. Mammography (MMG), galactography and ultrasonography (US) showed no abnormalities. Contrast helical CT scan revealed numerous patchy enhancement in the upper-outer quadrant of the left breast. Histological examnation revealed invasive ductal carcinoma with a predominant intraductal component. The second case was a 36- year-old woman who had noticed a left breast mass 2.4 cm in diameter. MMG and US suggested a localized breast cancer. Contrast helical CT scan revealed numerous patchy enhancement throughout the mammary gland in addition to the index tumor near the nipple. Histopathological results indicated invasive ductal carcinoma with widely spread ELC. In these two cases, the extent of intraductal cancer also correlated well with the CT findings. Breast CT scan can be an effective tool for determining the extent of the intraductal component of breast cancer, which is an important factor when considering breast-conserving therapy.