The role of near infrared diffusive (NIR) light imaging as an adjunct to ultrasound in differentiating benign from malignant lesions was evaluated in 27 patients with cancers, apocrine metaplasia, fibroadenomas, radial scar and ductal hypoplasia, cysts and normal tissues. The NIR functional imaging parameters used were deoxygenation and blood volume. Three out of four biopsy confirmed benign lesions found suspicious by ultrasound did not show significant and/or consistent deoxygenation and blood volume changes. Three out of four biopsy confirmed malignant lesions found suspicious for malignancy by ultrasound showed significant and consistent deoxygenation and blood volume changes. One complicated biopsy confirmed benign radial scar and ductal hypoplasia showed architectural distortion on screening mammogram and an isolated area of prominent ductal structure in ultrasound image. Optical functional imaging showed no consistent deoxygenation and blood volume changes. Optical imaging did not produce any false positives in obvious normal tissues and cysts diagnosed by ultrasound. These preliminary results indicate that optical imaging has a great potential to aid ultrasound in differentiating benign from malignant lesions and therefore to reduce the unnecessary biopsies. Ultrasound is frequently used in conjunction with mammography to differentiate simple cysts from solid lesions. When all criteria for a simple cyst are strictly adhered to, the accuracy of ultrasound is 96%-100% . However, the use of sonography to distinguish solid nodules has not yet been widely accepted . The considerable overlap of benign and malignant lesion characteristics disappointed many radiologists that they recommended biopsies should be performed on all solid nodules [3-12]. This results in a large number of unnecessary biopsies (currently 70% to 80% of biopsies are normal ). Goldberg et al. have speculated the cause of lack acoustic contrast between benign and malignant lesions and suggested that it may be related to the fact that even when tissues are pathologically different, their ultrasound properties are quite similar . More recently, the imaging capability of ultrasound equipment has been markedly improved. Stavros et al. have revisited the sonography issue and evaluated the current ability to distinguish benign from malignant solid breast lesions with ultrasound . In , a total of 750 palpable solid breast lesions were studied. Despite the known overlap features in some lesions, ultrasound was able to correctly classify 123 of 125 malignant lesions as intermediate or malignant. In this paper, we introduce for the first time the use of optical imaging as an adjunct to ultrasound in differentiating benign from malignant lesions. We demonstrate that optical contrast between benign and malignant lesions can provide more diagnostic information to further classify the acoustic intermediate group, thereby improving ultrasound specificity. Recently, optical diagnostics based on diffusing near infrared (NIR) light, have been employed in breast cancer detection [16-22. Functional imaging with NIR light is made possible in a spectrum window that exists within tissues in the 700 - 900 nm NIR region, in which photon transport is dominated by scattering rather than absorption (see Figure 1). To a very good approximation, NIR photons diffuse through relatively thick tissues, such as several cms of a human breast. Functional imaging with NIR light offers several novel tissue parameters that differentiate tumors from normal breast tissue. For example, hemoglobin desaturation in tumors may be increased due to the high oxygen demand of cancers , and blood volume may be increased over that of normal background tissue due to the metabolic needs of cancers. In addition, light scattering properties of the tumor may be enhanced due to the increased mitochondria population with respect to the background of normal cells , water concentration may be different in normal and cancer cells , and concentration of exogenous contrast agents allows tumor identification by observing the delivery of contrast agents through the permeable vascular beds to the extravascular space surrounding the tumor . In addition, NIR optical techniques have the great potential to provide information about other fundamental biological characteristics of tumors such as mitochondrial membrane potentials, intracellular calcium levels, and plasma membrane potentials. While NIR imaging provides novel optical contrast to improved tumor specificity, its relatively low resolution makes it unsuitable for morphological diagnosis. Its resolution is intrinsically limited by the diffusive nature of NIR light in tissue. Currently, optical imaging can distinguish simple structures of approximately 0.6 to 1 cm in size; sharp edges are typically blurred by a few millimeters . By utilizing complementary features of ultrasound and NIR imaging, i.e., ultrasound imaging capability and optical contrast, the combined diagnostic can overcome the deficiencies of either imaging technique [25-26J. This study reports the clinical evidence that low resolution NIR imaging can be used as an adjunct tool to improve tumor specificity and to reduce the unnecessary biopsies.