PURPOSE: We evaluated the MRI findings that suggested the presence of a residual cancer after a mammotome biopsy in pathologically proven breast cancer patients and the usefulness of MRI to diagnose a residual cancer and additional lesions. MATERIALS AND METHODS: We reviewed 41 breast cancer patients that underwent an ultrasonography-guided mammotome biopsy for complete resection of a breast lesion. MRI was performed for preoperative assessment and MRI findings suggestive of a residual cancer at the procedure site were analyzed and correlated to the pathological findings. Additional enhancements on breast MRI were analyzed, and the diagnostic accuracy of MRI for occult additional lesions was calculated. RESULTS: A total of 32 (78.0%) patients had a residual tumor. A mass was the most common MRI finding that suggested a residual cancer. Thick rim enhancement or a mass with a non-mass like enhancement were the most suspicious findings that suggested the presence of a residual cancer. The sensitivity, specificity and accuracy of MRI for the detection of a residual cancer were 81.3%, 66.7% and 78.0%, respectively. Additional malignant lesions were found in 7 cases. The sensitivity, specificity and accuracy of MRI for the detection of additional lesions were 100%, 60.0% and 76.5%, respectively. CONCLUSION: Further complete surgery should be performed, as residual tumors are found in 50% of the negative MRI examinations, whereas preoperative MRI is helpful to evaluate occult additional lesions.