PURPOSE: We wanted to evaluate the sonographic features of adenosis tumor of the breast. MATERIALS AND METHODS: We evaluated the sonographic findings of 9 lesions in 9 patients who had a diagnosis of adenosis tumor after excision. The mean age of the patients was 32 years. All the masses were palpable. The sonographic features were interpreted by the ACR-BI-RADS (American College of Radiology Breast Imaging Reporting and Data System). Three of the women underwent mammography. RESULTS: The mean size of the lesion was 2.4 cm, and the shape of the masses was oval (n=9). The orientation was parallel (n=9). The margin was circumscribed (n=7), angular (n=1) and microlobulated (n=1). The lesion boundary was an abrupt interface (n=9). The echo pattern was hypoechoic (n=8) and isoechoic (n=1). Posterior acoustic enhancement was seen in 7 masses, and the other 2 masses showed combined shadowing and enhancement. The final assessment categories were classified into category 3 (n=7) and category 4 (n=2). Mammograms performed in 3 patients revealed a circumscribed mass in 2 patients and negative findings in 1 patient. CONCLUSION: Adenosis tumor of the breast usually shows as an oval, parallel oriented, circumscribed, hypoechoic mass with posterior acoustic enhancement on sonography.