Fat necrosis is a rare lesion of the breast but is of clinical importance because it produces a mass (often accompanied by skin or nipple retraction) that is usually indistinguishable from carcinoma even with imaging studies. In the past, most fat necrosis has been seen after trauma, but now fat necrosis is commonly seen after fat injections to augment breast size or fill defects after breast surgery. The resultant mass may be confused with cancer. If untreated, the mass gradually disappears. If imaging is not typical of fat necrosis, the safest course is to obtain a biopsy. Core needle biopsy is usually adequate. Fat necrosis is also common after segmental resection, radiation therapy, or flap reconstruction after mastectomy.