BACKGROUND Metaplastic carcinoma of the breast (MPCB) is a rare breast cancer. We reviewed sonographic findings for MPCB. METHODS Grayscale ultrasonography (US), color Doppler US (CDUS), and spectral Doppler US (SDUS) findings for 10 patients with MPCB breast were retrospectively reviewed. RESULTS The prevalence of MPCB was 3.9% among cases of breast cancer in our hospital. All patients had a rapidly growing palpable breast mass. The mean lesion size was 5.7 cm. On US, the lesion shape was most commonly gently lobulated (90%); only one showed an irregular shape (10%). The lesion shape was most commonly circumscribed (90%). Nine tumors had an abrupt boundary and one had an indistinct boundary. Lesion echogenicity was hypoechoic and very hypoechoic (40%), hypoechoic (30%), or very hypoechoic, hypoechoic, and hyperechoic (30%). All our cases had cystic parts with posterior acoustic enhancement, representing necrosis or hemorrhage. CDUS showed peripheral, central and marginal color flow signals. The resistivity index (RI) of tumor vessels in the lesions ranged from 0.7 to 1.3. The axillary lymph nodes were enlarged on US and were positive for metastasis in three cases (30%). CONCLUSION MPCB is a rare rapidly growing tumor. US findings included gently lobulated, complex mass lesion with cystic parts and posterior acoustic enhancement, representing necrosis or hemorrhage. Increased color flow signals and relative high RI of the feeding arteries were also seen.