The morbidity and the mortality of breast cancer in Japan continue to increase year by year, making early detection very important. It is easy to detect microcalcifications on mammography, but they are more difficult to detect on breast ultrasound, which is said to be a limitation of breast ultrasound. Microcalcifications are found in ductal carcinoma in situ (DCIS), which is the earliest stage of breast cancer. We have developed a new technology, the microcalcification detection filter (firefly), and examined diagnostic and clinical application. It is often difficult to identify the high-intensity echogenic spots of microcalcifications on the monitor because the mammary gland has a very complicated structure on B-mode ultrasound. Using the theory of CFAR (Contrast False Alarm Rate), we have erased the echogenic breast tissue on screen and emphasized punctate high-intensity microcalcifications. The microcalcifications can be identified with this filter, like the glitter of fireflies on a dark night. Two-hundred and eighteen cases were examined, in whom a clear increase in detectability was found. Intervention under stereo (mammography) guide is the mainstay of diagnosis of microcalcifications, but it can be performed only at a limited number of institution. The development of this filter will allow biopsy under guided ultrasound at any hospital, and it will also be possible to observe the microcalcifications on sampling. Furthermore, the extent of the surgical margin needs to be accurately evaluated before surgery. This filter will enable mapping of malignancy at the same position as the operation. Breast cancer screening with ultrasound may become widespread with use of the “firefly” microcalcification detection filter. The significance and utility of “firefly” are great.