Purpose: The purpose of this study is to evaluate the use of second-look ultrasonography (US) for investigating additional suspicious lesions detected on preoperative staging magnetic resonance imaging (MRI) for breast cancer. Methods: Between September 2008 and August 2010, 1,970 breast MRIs were performed at our medical institution for the evaluation of breast cancer before surgery. Second-look US was recommended for 135 patients with 149 suspicious lesions, following the MRI interpretation, and 108 patients with 121 lesions were included in this study. The detection rate on second-look US, according to the lesion type, diameter, and histopathological outcome, was analyzed. Results: Of the 121 lesions considered in this study, 97 (80.2%) were diagnosed on MRI as masses and 24 (19.8%) as non-mass-like lesions; 105 lesions (86.8%) were correlated and 16 (13.2%) were not correlated with the findings of second-look US. Of the 105 correlated lesions, 29 (27.6%) were proven to be malignant and 76 (72.4%) were benign. Although a greater number of large malignant lesions were correlated on second-look US than small benign lesions, there was no statistically significant difference according to lesion diameter or type, as seen on MRI or pathology. Conclusion: We have concluded that second-look US is a useful diagnostic tool for lesions incidentally detected on breast MRI, as in this study, it could identify 86.8% of the MRI-detected breast lesions.