Non-mass abnormalities are of five subtypes, i.e., abnormalities of the ducts, hypoechoic areas in the mammary gland, architectural distortion, multiple small cysts, and echogenic foci without a hypoechoic area. The main lesions recognized as non-mass abnormalities include ductal carcinoma in situ (DCIS), fibrocystic change, and intraductal papilloma. In the case of DCIS with a non-mass abnormality, a hypoechoic area in the mammary gland is the most frequent finding at 76%, followed by abnormalities of the ducts at 16%. Architectural distortion is found in about 2% of DCIS; however, it is an important finding because it is often observed in small invasive carcinoma and invasive lobular carcinoma. In this article, we explain the five classifications and present some cases.