In breast cancer, we often find it difficult to accurately identify the cancer margin due to intraductal spread, especially in patients with ductal carcinoma in situ. In such cases, we perform mastectomy for complete resection of the cancer. If we are able to accurately detect intraductal spread, we can safely perform partial resection without remaining tumor, and therefore potentially decrease the number of mastectomy patients. In our institution, we routinely perform MRI and contrast-enhanced ultrasonography (CEUS) in breast cancer patients to evaluate intraductal spread; we have found that CEUS can also accurately detect intraductal spread. In this article, we report a case of a partial resection in a patient with ductal carcinoma in situ in whom intraductal spread was clearly visible using CEUS. In ductal carcinoma in situ (DCIS), we often find it difficult to accurately define the peripheral margin because of intraductal spread. In such cases, we perform mastectomy for complete resection of the cancer. If we are able to accurately detect the intraductal spread of DCIS, we can safely perform partial resection without remaining tumor, and we can potentially decrease the number of mastectomy patients. MRI is commonly used for the detection of intraductal spread, and we have found that contrastenhanced ultrasonography (CEUS) can also accurately detect the same. We report a case of DCIS in whom we performed partial resection after precisely visualizing the cancer margin with intraductal spread using CEUS. A 38-year-old woman was admitted to our hospital with a lump in the upper lateral region of her right breast. Mammography revealed a focal asymmetric density in the upper region of her breast, and ultrasonography showed a 20 · 10-mm low-echoic mass. DCIS was pathologically confirmed by needle biopsy. Contrast-enhanced MRI showed the tumor and intraductal spread located toward the nipple (Fig. 1). CEUS also showed the tumor and intraductal spread (Fig. 2). The total tumor size was 45 mm, and CEUS detected 2to 3-mm intraductal spread lesions that were undetectable on MRI. Although mastectomy is a common treatment for tumors of this size, the patient wanted to preserve her breast. Therefore, we performed partial resection after accurate ultrasound diagnosis of the intraductal spread. Pathologically, the margin of the excised specimen was negative (the final margin of the tumor was 7 mm) and complete resection was performed. The enhanced area detected on CEUS corresponded with the pathological lesion (Fig. 2). The lesion was staged as Tis N0 M0 stage 0. The patient underwent radiotherapy and is alive without recurrence for 24 months. When partial resection for breast cancer with intraductal spread is performed, it is necessary to