We report the case of a 39-year-old asymptomatic woman who presented for screening mammography. Mammography revealed a round, partially circumscribed, partially indistinct, noncalcified 4.5 cm mass in upper outer right breast. This vaguely palpable mass appeared as a well-circumscribed, oval, hyperechoic, parallel, septated mass on sonography. Ultrasound-guided core biopsy led to the diagnosis of hibernoma. Few cases of hibernomas have been reported in the English literature, and their occurrence in the breast is particularly rare [Eur J Surg Oncol 26 (2000) 430; Am J Surg Pathol 25 (2001) 809]. To our knowledge, this is the first case of mammary hibernoma in which the ultrasonographic and mammographic features of this unusual entity are demonstrated.