Tensile testing to failure was done on 235 cortical specimens that had been machined from forty-seven femora from human cadavera. The donors had ranged in age from twenty to 102 years at the time of death. After mechanical testing, the porosity, mineralization, and microstructure were determined. Linear regression analysis showed that the mechanical properties deteriorated markedly with age. Ultimate stress, ultimate strain, and energy absorption decreased by 5, 9, and 12 per cent per decade, respectively. The porosity of bone increased significantly with age, while the mineral content was not affected. Microstructural analysis demonstrated that the amount of haversian bone increased with age. Both bivariate and multivariate analyses demonstrated the importance of age-related changes in porosity to the decline in mechanical properties. Changes in porosity accounted for 76 per cent of the reduction in strength. Microstructural changes were highly correlated with porosity and therefore had little independent effect. Mineral content did not play a major role. Thus, the quantitative changes in aging bone tissue, rather than the qualitative changes, influence the mechanical competence of the bone.