Ageing changes in the normal human female breast were studied to determine their significance for the evolution of mammary cancer. Employing the morphometric techniques of point counting and planimetry, objective quantitative measurements were made of the structure of the normal female breast in 58 subjects from the prepubertal to late postreproductive period. The relative amounts of epithelial and connective tissue varied with age, and the epithelial elements (combined lobular and extralobular) were unevenly distributed within the gland, with lower containing more than upper quadrants. The upper outer quadrant, however, usually contained the largest proportion of lobular units, which may relate to the higher incidence of lobular carcinoma found in this quadrant. Involution was shown to be a premenopausal rather than postmenopausal phenomenon. Mammary dysplastic changes were uncommon in all age groups.