The authors conducted a case-control study at two Boston, Massachusetts, are hospitals to evaluate the relation of anatomic features of the breast, visible on the xeromammogram, to the risk of breast cancer. The cases were 408 women with newly diagnosed breast cancer and the controls were 1021 women without signs or symptoms of breast disease. The features of the breast assessed were the "parenchymal pattern" as defined by Wolfe (Am J Roentgenol 1976; 126:1132-9), and specific radiologic characteristics which are components of the parenchymal pattern classification. Women whose mammogram showed the P2 or DY parenchymal patterns were at elevated risk compared to women with the N1 pattern. Further, risk increased regularly with increases in the percentage of the breast that showed nodular densities and with increases in the average size and concentration of these densities. Women with extensive homogeneous density were also at elevated risk. These findings were observed only among women aged 20-59 years. In this group, women with nodular densities in 60% or more of the breast appeared to have a five-fold increase in risk compared to women without nodular densities in the breast.