Women with dense breasts have an increased risk of developing breast cancer and of missed breast cancers on mammography (MG) [1-5]. Several states in the USA have passed a breast density notification law, which requires radiologists to inform a woman if dense breast tissue is found by MG so that she can be offered additional screening with ultrasound (US) or magnetic resonance imaging [6]. Although the benefits of supplemental screening for dense breasts in terms of mortality or morbidity reduction are still not proven [7], there are significant efforts on-going in this direction [8; 9]. Thus, a non-ionizing and cost-effective breast density assessment for breast cancer risk evaluations and further imaging stratification of women is of increasing interest. To assess the feasibility and diagnostic accuracy of a novel hand-held ultrasound (US) method for breast density assessment, which measures the speed of sound (SoS), in comparison to the ACR mammographic (MG) categories.