A technique for measuring thein vivoaverage velocity of ultrasound in the human female breast was developed and applied to determine the range of such values in the breast of normal subjects in the approximate age ranges 20 to 80 years and in subjects with selected breast pathologies. Reasonable correlation of the velocity values with breast tissue type, as determined by x ray (mammography), was obtained, indicating that ultrasonic velocity data provide useful information on the main tissue constituents in the breast and that this technique should allow safe, long‐term study of changes that may occur in the composition of such tissues.