Fat necrosis is well depicted on imaging by characteristic findings such as oil cysts. However it may be rarely indistinguishable from carcinoma clinically and radiologically, and require biopsy for diagnosis. We report a case of fat necrosis simulating malignancy following reduction mammoplasty. The screening mammogram of a 63-year-old woman, who had received bilateral reduction mammoplasties 10 months previously, showed an irregular high-density lesion with a spiculated margin in the upper inner quadrant of the left breast. Ultrasonography revealed a 1.5 cm sized, ill-defined, and irregular hypoechoic lesion with posterior acoustic shadowing. The confirm diagnosis was fat necrosis, by ultrasonography-guided core needle biopsy. On the 6-month follow-up ultrasonography, the lesion was found to have decreased in size.