Purpose: The high breast density is one of the biggest risk factors for breast cancer. Identifying patient having persistent high breast density is important for breast cancer screening and prevention. In this work the authors propose for the first time an x-ray phase-shifts-based method of breast density measurement. Methods: When x ray traverses the breast, x ray gets not only its intensity attenuated but also its phase shifted. Studying the x-ray phase-shifts generated by the breast tissues, we derived a general formula for determining the volumetric breast density from the breast phase map. The volumetric breast density is reconstructed by retrieving the breast phase map from just a single phase-sensitive projection of the breast, through the use of an innovative phase retrieval method based on the phase-attenuation duality. In order to numerically validate this phase-shifts-based method for measuring the volumetric breast density, the authors performed computer simulations with a digitally simulated anthropomorphic breast phantom. Results: Using the proposed phase-shifts-based method, we reconstructed the breast phantoms volumetric breast density, which differs from the phantoms intrinsic breast density by only 0.06. In the presence of noises in the projection image, the reconstructed volumetric breast density differs from the phantoms intrinsic breast density by only 1.79 for a projection signal-to-noise-ratio (SNR) of 34. The error in reconstructed breast density is further reduced to 1.61 and 1.55 for SNR 68 and SNR 134, respectively, achieving good accuracies in the breast density determination. Conclusions: The authors proposed an x-ray phase-shifts-based method of measuring the volumetric breast density. The simulation results numerically validated the proposed method as a novel method of breast density measurement with good accuracies. © 2012 American Association of Physicists in Medicine.