A 15-year prospective study of mammographic parenchymal patterns in 7,123 women over 30 with 658 prevalent and 131 incident primary breast neoplasms demonstrated that no pattern is a reliable indicator of initial or developing cancer. Cancer in glandular breasts is less frequent, more difficult to detect by mammography, and tends to remain prevalent compared with the more frequent tumors in fatty breasts. As women get older, their breasts assume similar patterns and the incidence of both prevalent and incident tumors increases. Parenchymal patterns cannot signify which breasts may remain glandular, nor which women may be at risk of cancer.