Fragility fractures at the trochanter (TR) and the femoral neck (FN) have distinct etiologies, but the underlying age-related structural changes at these proximal femoral sub-regions are poorly understood. 28 young (41+/-3 years) and 124 elderly (74+/-3 years) healthy Caucasian women underwent volumetric quantitative computed tomography at the hip. Integral (i), cortical (c) and trabecular (t) bone mineral density and content (BMD, BMC) were measured. Geometric parameters included cross sectional area (CSA), and volumes of the integral, cortical and trabecular regions (VOL). Structural measures included indices of compressive (Compstr) and bending (BSI) strength. After adjusting for height and weight, an F-test was used to compare the TR and the FN mean values between young and elderly and to test for interaction to compare logarithmic difference of young and elderly (log(Young)-log(Elderly), Y/Ed) between the FN and the TR in an ANOCOVA model. All BMC, iBMD and tBMD values were significantly lower in elderly than in young women, with the largest Y/Ed in the FN tBMC and tBMD (P<0.0011 and P<0.0001). cBMD in young and elderly groups was not significantly different at the TR while at the FN it was greater (P=0.0075) in elderly than young women, showing significant Y/Ed (P=0.0003) dependence on skeletal site. Elderly women had significantly larger iVOL and CSA values (0.0001<P<0.0051), except for the FN iVOL. cVOL values were smaller in elderly than young women (P<0.0001). Y/Ed in bone geometry differed by sub-region only for cVOL measures (P=0.0267). Despite larger CSA and iVOL measures in elderly, the younger women had greater Compstr (P<0.0001) and BSI (P=0.0051). Thus, although both the TR and the FN appear to increase in size with age, this enlargement is insufficient to protect against loss of bone strength.