BACKGROUND: A reduction in local residual breast cancer is important to avoid local recurrence after breast conserving treatment (BCT). Previous studies suggested the usefulness of contrast-enhanced computed tomography (CE-CT) in detecting extensive intraductal component(EIC), which is associated with local recurrence. This study investigates the practical role of CE-CT in BCT. METHODS: In 130 women whose breasts were examined by CE-CT preoperatively, intraductal spread detected using ultrasound (US), mammography (MMG) and CE-CT, and EIC detected by histological examination were analyzed to determine possible correlations among the extent and subtypes of intraductal components. RESULTS: The sensitivities of EIC detection by US, MMG and CE-CT were 29%, 53% and 88%, respectively, and the corresponding specificities were 58%, 70% and 72%, respectively. In 6 patients, EIC could only be visualized using CE-CT. CE-CT demonstrated the extent of cancerous lesions more accurately than standard diagnostic modalities. Intraductal components with a higher histological grade tended to show stronger enhancement. Most false negative intraductal components obtained on CE-CT (6/7) were of a low histological grade. CONCLUSIONS: CE-CT is useful for visualizing EIC and small invasive foci of breast cancer, and can suggest the optimal extent of resection.