Abstract We describe a new method for assessing the skeletal Ca/P ratio in vivo using X-ray absorptiometry. By placing cerium (Ce) and samarium (Sm) filters in the X-ray beam from a commercial X-ray source (Norland), mean photon energies of 39 and 89 keV were obtained. The instrument was designed to take measurements of the forearm, at a site located at the distal 1/3 of the radius. The system was calibrated with three bone phantoms: Ca 10(PO 4) 6(OH) 2, Ca(HPO 4)(H 2O) 2 and Ca(HPO 4) 2(H 2O)). The precision for measuring the Ca/P ratio in the human radius was 2.3% CV for a skin dose to the forearm ranging from 0.3 to 0.4 mGy, depending on the width of the arm. The Ca/P ratio of the radius was significantly lower in patients with postmenopausal osteoporosis than in premenopausal controls.