Evaluation of a mastectomy site is more effective with ultrasonography (US) than with either mammography or chest computed tomography because abnormalities are usually small and close to the skin surface. US does not involve the use of ionizing radiation and has a multiplanar scanning capability. The technique is readily available and inexpensive, and it allows real-time monitoring of needle tip placement during biopsy of a lesion. Normal US anatomy of the chest wall after mastectomy usually consists of four layers: skin, subcutaneous fat, pectoral muscles, and rib and intercostal muscle. The axilla is changed in appearance after lymph node dissection, but it remains the same in patients who have undergone simple mastectomy. US can accurately depict benign and malignant conditions in the mastectomy site, including fluid collection, fibrosis, local recurrent tumor, and metastatic lymphadenopathy, and can enable accurate diagnosis based on findings at fine needle aspiration biopsy. © RSNA, 2004.