The aim of the current work was to quantify the ultrasonic properties of the whole breast in vivo as a function of age. Forty-four women were scanned using a computerized ultrasonic scanner developed in our laboratory. Raster scans in two orthogonal views, mediolateral and craniocaudal, were obtained using the ultrasonic through-transmission method. By combining the information from the two views, we estimated two acoustic properties: speed of sound and attenuation coefficient. On the basis of the results, both the attenuation coefficient and the speed of sound follow a three-phase age-related pattern. During the first phase, which corresponds to ages 20 to 35 y, both properties decrease with time and then remain roughly unchanged until about 55 y. During the third phase corresponding to ages >55 y, values decrease again with time. The mean speed of sound decreases from 1504 ± 35 m/s at <30 y to 1452 ± 9 m/s at >60 y (p < 0.01), and the attenuation coefficient decreases from 1.27 ± 0.32 to 0.96 ± 0.13 dB/cm/MHz (p < 0.03), respectively. In conclusion, both the ultrasonic speed of sound and the attenuation coefficient of breast tissue are age related. Both parameters decrease during life, markedly during the first and third phases. These changes may be attributed to anatomic and physiologic changes associated with reproductivity and menopause.