A 62-year-old woman in whom ductal carcinoma in situ (DCIS) was incidentally detected in a screening examination.An irregular shape, indistinct margin, and hypoechoic mass (arrows) were noted in the left breast on ultrasonography. No microcalcifications were present. After left partial mastectomy, non-comedonecrosis-type pure DCIS was confirmed. Both the estrogen and progesterone receptors were positive, and human epidermal growth factor receptor was negative on immunochemical staining. The Ki-67 index was less than 20%.