A case-control study was conducted to evaluate the association of body size with morphologic features of breast tissue visible on mammograms, and to analyze the interrelations of these factors with breast cancer risk. The cases were 362 women with newly diagnosed breast cancer identified in 1978-1979 in three large hospital-based xeromammography units in Boston, Massachusetts, and one unit in Livingston, New Jersey. The controls were 686 women referred to these units in the same period for a "routine" mammogram. The parenchymal pattern (N1, P1, P2, DY) and the per cent of the breast showing nodular densities were the principal mammographic features assessed. Among controls, body weight was strongly but inversely associated with the per cent of women who had the high risk P2 or DY patterns and with the mean per cent of the breast showing nodular densities. Body weight and the amount of nodular densities were both directly related to breast cancer risk. The strengths of the relations of body weight and of nodular densities to risk were each increased when the other factor was taken into account.