PURPOSE To describe the ultrasonographic (US) features of adenoma in a lactating breast. MATERIALS AND METHODS The US scans of 11 lactating adenomas in nine patients aged 25-36 years examined in 1991-1996 were retrospectively reviewed. In all lesions, diagnoses were cytologically or histologically proved after US-guided fine-needle aspiration biopsy (eight lesions) or core biopsy (three lesions). Six patients were lactating, and three patients were in the third trimester of pregnancy. The US features analyzed were shape, orientation to the chest wall, border characteristics, echogenicity, homogeneity, enhancement or shadowing, and depth within the breast. RESULTS Most lesions had benign US features such as ovoid shape with the long axis parallel to the chest wall (10 of 11), well-defined margins (eight of 11), homogeneous echotexture (eight of 11), and posterior acoustic enhancement (10 of 11). Four lesions had US features that resembled malignant lesions, such as irregular margins, heterogeneous echotexture, and posterior acoustic shadowing. CONCLUSION The US features, although generally benign, are not specific. Tissue sampling with fine-needle aspiration biopsy is recommended. Core biopsy is necessary if a diagnosis cannot be made with the aspirate but is not performed initially because of the possibility of milk fistula formation.