US-guided large-gauge core needle biopsy has replaced fine-needle aspiration and excisional biopsy for sampling suspicious breast lesions. The core needle biopsy has many advantages: it is relatively simple, it can be done with local anesthesia, it enables accurate targeting under sonographic guidance and it leaves no scar. However, one rare disadvantage of this technique is the possible seeding of malignant cells along the needle track. We report here on a case of gross track seeding in a 76-year-old woman, and this was observed 70 days after performing a 14-gauge core needle biopsy.