Real-time virtual sonography® (RVS) is a new mode ultrasound (US) diagnostic system that registers and displays real-time ultrasound images together with corresponding previously acquired CT or MRI images. The clinical advantages of the RVS system for abdominal imaging have are well established and routinely used in the clinic. We extend the RVS system for breast imaging using a high-frequency linear array probe and establish its clinical advantages. Breast CT has been gaining popularity for preoperative evaluation of malignant breast lesions ever since the introduction of multislice CT in Japan. Because high quality three-dimensional reconstruction CT images are acquired in the same supine position as the surgical treatment, it also serves as a valuable simulation tool for breast conservative therapy. However, the correspondence between displayed static CT images and the real field of operation requires visual approximate estimation. Real-time ultrasound would be the modality of choice for overcoming the disadvantage of helical CT in that it provides feedback on which the clinician can base dedcisions. However, ultrasound′s accuracy in detecting and localizing lesions tends to be inferior to that of CT and, in addition, requires an experienced examiner. We attempt to apply RVS to overcome these limitations for breast imaging fusing three dimensional CT data and real-time ultrasound. Our RVS is not limited to CT alone, but can be extended to MRI and other modalities. Although the effect of difference in positioinit varies significantly between MRI and US, our RVS results obtained using the registration algorithm commonly used in CT show promise. We expect that breast RVS combined with MRI data should prove effective as an assistant tool in second-look ultrasound for MR-detected lesions.