Breast density, as visible on mammograms, is generally assessed as the occupied percentage of the breast and is a risk factor for breast cancer. Various studies have looked into the causation and alteration of relative density but the relation of a determinant with a relative measure does not allow a direct etiologic interpretation. It was our goal to compare the effects of known determinants on relative density and the absolute amounts of dense and nondense tissues. We measured the absolute and relative densities in a population of 418 postmenopausal women participating in a breast cancer screening program. The occupied surface area was calculated after manually tracing the contours of the tissues on digitized mammograms. Information on determinants was available through physical examination and questionnaires. Data were analyzed by multivariate linear regression. Age and parity were found to decrease the amount of dense tissue and the ages at menarche and menopause were found to increase it (R(2) = 13%). The amount of nondense tissue was increased by higher body mass index (BMI), age, and parity (R(2) = 43%). Relative density was affected by a combination of these factors (R(2) = 29%) with directionalities of effects that are comparable to those of dense tissue. However, the magnitudes of these effects were the resultant of the effects on dense and nondense tissues. The influence of BMI on relative density was completely due to an effect on nondense tissue. Although relative density is a relevant prognostic factor, inferences about the etiology of breast density should be made on the basis of absolute measures.