Near-infrared spectroscopic tomography was used to measure the properties of 24 mammographically normal breasts to quantify whole-breast absorption and scattering spectra and to evaluate which tissue composition characteristics can be determined from these spectra. The absorption spectrum of breast tissue allows quantification of (i) total hemoglobin concentration, (ii) hemoglobin oxygen saturation, and (iii) water concentration, whereas the scattering spectrum provides information about the size and number density of cellular components and structural matrix elements. These property data were tested for correlation to demographic information, including subject age, body mass index, breast size, and radiographic density. Total hemoglobin concentration correlated inversely to body mass index, likely because lower body mass indicates proportionately less fat and more glandular tissue, and glandular tissue contains greater vascularity, hence, more total hemoglobin. Optical scattering was correlated to breast diameter, subject age, and radiographic density. In the radiographic density, fatty breasts had low scattering power and extremely dense breasts had higher values. This observation is consistent with low attenuation of conventional x-rays with fat and higher attenuation in glandular tissues. Optically, fatty tissues have large scatterers leading to a low scattering power, whereas glandular or fibrous tissues have more cellular and collagen-based structures that lead to high scattering power. The study presents correlative data supporting the hypothesis that optical measurements of absorption and scattering can provide physiologically relevant information about breast tissue composition. These breast constituents vary significantly between individuals and can be altered because of changes in breast physiology or pathological state.T he breast is a highly heterogeneous and dynamically complex organ whose characteristics depend on factors such as age, hormonal status, habitus, family and medical history, and genetics (1-3). Normal breast tissue changes considerably during development, pregnancy, and menopause, and throughout the menstrual cycle. It has been documented that blood flow can increase up to 50% at the time of ovulation; and by the end of the monthly cycle, some women may undergo breast enlargement of up to 20% because of increased vascularity and water content (1). Total blood content can vary up to a factor of 5 or more between women based on their body-fat content, making the breast one of the most physiologically variable tissues in the human body. With nearinfrared (NIR) spectral imaging, hemoglobin concentration, oxygen saturation, and water content can yield information about the current physiological state of the normal breast.The relatively good transparency of tissue in the red and NIR spectrum (i.e., 600-1,000 nm) permits sufficient light penetration to detect signals through as much as a dozen centimeters of the breast. In the NIR spectrum, the primary absorbers of light are hemoglobin, oxyh...