We have assessed the agreement of radiologists who quantitatively estimated the extent of various mammographic features of the breast. These features are of potential relevance to the estimation of the future risk of developing breast cancer. After preparation, in which various mammographic appearances and their classification were discussed and agreed upon, two radiologists, experienced in mammography, independently estimated the extent of various types of radiological density in 120 sets of mammograms, each comprised of two mediolateral and two craniocaudal views. The results showed a high level of agreement between the two radiologists in the classification of total mammographic densities (r = 0.89). For classification of specific types of density, agreement was best for homogeneous densities (r = 0.79), more intermediate for nodular densities (r = 0.71) and worst for linear densities (r = 0.48). These results show that radiologists can, with suitable preparation, identify total breast density, a mammographic sign relevant to a woman's risk of later developing breast cancer, with a high degree of agreement.