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Bone undergoes structural changes with aging, but the nature of qualitative changes remains to be established. Blocks of midshaft femur were taken at autopsy from men of four different age groups: 20-25 years, 40-45 years, 60-65 years, and 80-85 years. Each femoral specimen was analyzed by density fractionation, a technique that allows the separation of bone by extent of mineralization and maturity. In the 20-25 group, lower density bone predominates. The 40-45 group is characterized by more highly mineralized bone with an increase in the 2.1-2.2 g/cc fraction. At 60-65 years, an increase in the lower density fraction was found, indicating an increase in new bone formation. At 80-85 years, there is an increase in the highest density bone (2.2-2.3 g/cc), which may represent regions of interstitial bone not properly removed through remodeling processes. Chemical studies did not reveal any change in Ca, P, Ca + PO4, or Ca/P molar ratio with respect to age. X-ray diffraction studies show no changes in apatite crystal size with respect to age or degree of mineralization. Morphological studies documented increased remodeling activity and endosteal trabecularization in the older age groups, as well as increased intracortical porosity. An increase in the highest density fraction with aging may represent a pool of bone mineral that is less accessible to remodeling, which may be the interstitial bone.