Bone mineral content (BMC) of the femoral neck and shaft was determined with dual photon absorptiometry, using 153Gd. Comparison of BMC with the amount of hydroxyapatite (HA) of in vitro specimen showed correlation coefficients of 0.992 and 0.996 for the femoral neck and shaft respectively. In the femoral neck the amount of cortical bone in a bone section varies from 16% ash weight in the proximal part to 71% in the distal part. Corresponding to the site of BMC measurements, the cortical bone constitutes 57% in the femoral neck and 95% in the femoral shaft. The precision error of measurements of BMC in vivo, expressed as the coefficient of variation for repeated determinations, was 1.4% for the femoral neck and 1.3% for the femoral shaft. In the femoral neck it is possible to distinguish between structures consisting mainly of cortical bone and structures containing mostly trabecular bone. While the cortical bone value decreases only slowly with age in normal women, corresponding to BMC of the femoral shaft, the trabecular bone value decreases rapidly even compared with BMC of the femoral neck. Despite the significant correlation between the values for cortical and trabecular bone a distinction seems essential from a clinical point of view.