We investigated sonographic changes to the breast after imaging guided core breast biopsy. We studied 31 breast lesions in 29 patients before, immediately after, and 2 to 9 days after core biopsy looking for hematomas. We found sonographic changes to the breast consistent with fluid collections after core biopsy in seven of 31 breast lesions (23%). Of the biopsy sites with sonographic changes, evidence suggested that six of 31 (19%) likely had hematomas. Sonographic changes after core breast biopsy are common and the formation rate of suspected hematomas is greater than previously believed although generally not clinically significant.