BACKGROUND Needle localization breast biopsy (NLBB) is the standard for removal of breast lesions after vacuum assisted core biopsy (VACB). Disadvantages include a miss rate of 0% to 22%, a positive margin rate of approximately 50%, and vasovagal reactions (approximately 20%). We hypothesized that clip migration after VACB is clinically significant and may contribute to the positive margin rates seen after NLBB. METHODS We performed a retrospective review of postbiopsy films in patients who had undergone VACB with stereotactic clip placement for abnormal mammograms. We measured the distance between the clip and the biopsy site in standard two view mammograms. The location of the biopsy air pocket was confirmed using the prebiopsy calcification site. The Pythagorean Theorem was used to calculate the distance the clip moved within the breast. Pathology reports on NLBB or intraoperative hematoma-directed ultrasound-guided breast biopsy (HUG, which localizes by US the VACB site) were reviewed to assess margin status. RESULTS In all, 165 postbiopsy mammograms on patients who had VACB with clip placement were reviewed. In 93 evaluable cases, the mean distance the clip moved was 13.5 mm +/- 1.6 mm, SEM (95% CI = 10.3 mm to 16.7 mm). Range of migration was 0 to 78.3 mm. The median was 9.5 mm. In 21.5% of patients the clip was more than 20 mm from the targeted site. Migration of the clip did not change with the age of the patient, the size of the breast or location within the breast. In the subgroup of patients with cancer, margin positivity (including those with close margins) after NLBB was 60% versus 0% in the HUG group. CONCLUSIONS Significant clip migration after VACB may contribute to the high positive margin status of standard NLBBs. Surgeons cannot rely on needle localization of the clip alone and must be cognizant of potential clip migration. HUG as an alternative biopsy technique after VACB eliminates operator dependency on clip location and may have superior results in margin status.