Tumor cell displacement is occasionally observed in surgical specimens after core needle biopsy. We report a case involving a 58-year-old woman in whom a microlobulated mass was detected on mammography, after which she underwent an ultrasonography- guided core needle biopsy. Thirty-six days later, a new low-echoic lesion was detected on ultrasonography, located from the original mass to the subcutaneous insertion site of the needle biopsy. Fifty days after the biopsy, breast-conserving surgery was performed. Histopathology showed a main tumor containing 2 components, namely invasive carcinoma of no special type, which showed tubule and gland formation, and invasive papillary carcinoma(WHO Classification). The invasive carcinoma with tubule formation was also observed in the adjacent collagen fibers, suggesting tumor displacement in the needle tracks. It has been suggested that malignant cells displaced by core needle biopsy do not survive. However, based on the present case, we recommend ascertaining the absence of extensive tumor displacement using ultrasonography unless all needle tracks are included in the planned resection area.