Measurements of bone density using dual-energy x-ray absorptiometry are generally based on the areal projection, which incompletely accounts for size. The larger areal bone density in older men compared with older women is primarily due to their larger bone size, conferring a biomechanical advantage that may be a major factor contributing to lower hip fracture rates. The aim of this study was to evaluate estimated volumetric bone density at the hip in men and women with and without fractures to better determine the role of estimated volumetric density vs. size in hip fracture risk. This prospective population-based study compared 852 women and 635 men without fractures with 73 women and 23 men with hip fractures. As expected, areal bone mineral density (BMD) and cross-sectional area were lower in women than men, and areal bone density was lower in those with hip fractures compared with nonfracture subjects. However, estimated volumetric BMD was the only parameter, apart from age, that was the same in women and men both without hip fractures (0.31 +/- 0.06 and 0.31 +/- 0.06 g/cm3, respectively) and with hip fractures (0.25 +/- 0.04 and 0.26 +/- 0.04 g/cm3, respectively). Using the World Health Organization 2.5 SD cut-off for osteoporosis for hip fracture prediction, estimated volumetric BMD was more sensitive than areal BMD in men (70 vs. 43%; P = 0.04) and similar to that in women, in whom sensitivity was similar for both areal (73%) and estimated volumetric (78%) BMD cut-offs. Thus, men and women have hip fractures at the same estimated femoral neck volumetric BMD, which is largely independent of the size artifact inherent in areal BMD. This aspect of estimated femoral neck volumetric BMD suggests that it can provide a single measure that could be used in men and women. It needs further exploration for a role in assessment of hip fracture risk across the sexes and particularly in men.