Epidemiologic evidence suggests that changes in breast density are related to changes in breast cancer risk. In collaboration with researchers at the University of Toronto and the Karmanos Cancer Institute in Detroit, Michigan, NCI investigators are using a novel ultrasound tomography scanner (UST) to define the time course of volumetric breast density changes among women receiving tamoxifen therapy. Tamoxifen is known to reduce breast density and breast cancer risk. Investigators are enrolling women receiving tamoxifen monotherapy for clinical indications at Karmanos Cancer Institute to undergo repeated volumetric breast density assessment with UST. They will assess whether UST examinations performed as early as one to three months following initiation of tamoxifen can identify women whose breast density is demonstrated to have declined at one year using mammography. If successful, this study will show that density changes related to tamoxifen occur rapidly, providing a clinical tool for assessing the efficacy of therapy.