Bone mineral density (BMD) measurement by hip dual-energy X-ray absorptiometry (DXA) is considered the best predictor of osteoporotic fracture risk. BMD takes into account only in part the bone cross-sectional area that is an important determinant of both bone compression strength and of bending breaking resistance. From DXA measurements of proximal radius (Osteoplan, NIM, Verona, Italy) we obtained the projected outer diameter (D) and the mean diameter of the medulla (d), by an algorithm based on the assumption of a constant cortical volumetric density of 1050 g/cm3. The algorithm was validated by the good correlation found (r = 0.8) between calculated d and that actually measured by peripheral quantitative tomography (pQCT; XCT 960, Stratec, Unitrem, Italy) at the same radial site. The D and d values were used to calculate a bending breaking resistance index (BBRI) that is a component of the cross-sectional moment of inertia. The BBRI measured in 5460 women aged 35-89 years, was stable up to the age of 65-70 years and rapidly declined thereafter by 0.7% per year. This profile appears to be due to the fact that the increase in medullary area is compensated in terms of mechanical resistance by enlargement of cross-sectional area. In 68 women with either previous femoral neck (n = 41) or pertrochanteric fracture (n = 27) DXA measurements at proximal and ultradistal radius, lumbar spine and femoral neck were obtained together with the evaluation of proximal radius BBRI. The diagnostic accuracy of BBRI was somewhat comparable to that of spine and femoral neck BMD and significantly superior to that of ultradistal and proximal radius BMD, from which it was derived. Despite the obvious limitation of the cross-sectional nature of this study, our results indicate that a simple re-elaboration of the data obtained by peripheral radial densitometry may achieve diagnostic accuracy for hip fracture risk assessment only marginally lower than that of the direct measure of the BMD of the femoral neck. They also give additional support to the view that bone geometry, particularly for compact skeletal segments, is a determinant of its strength at least as important as bone density.