Compressive testing to failure in the weight-bearing axis was done on 255 specimens of cancellous bone that had been machined from forty-four femora from human cadavera. The donors had ranged in age from twenty to 102 years at the time of death. After mechanical testing, the apparent density and trabecular architecture were determined. Linear regression analysis showed that the compressive strength decreased by 8.5 per cent each decade (p < 0.001). Apparent density and volume fraction also decreased significantly with age (p < 0.001). Histomorphometric analysis demonstrated that the surface-to-volume ratio and the mean separation of the trabecular plate increased with age, whereas the mean thickness and connectivity of the trabecular plate decreased. Both bivariate and multivariate analyses demonstrated that age-related changes in apparent density played an important role in the decrease in mechanical strength, accounting for a 92 per cent reduction. Microstructural changes were highly correlated with apparent density and therefore had little independent effect. Thus, similar to the situation with cortical bone, the quantitative changes in aging cancellous-bone tissue, rather than the qualitative changes, influenced the mechanical competence of the bone. CLINICAL RELEVANCE: This study provides information concerning the difference in the properties of human cancellous bone as a function of age. Because of the importance of changes in apparent density, non-invasive means can be used to estimate the mechanical properties of cancellous bone in vivo. Thus, it may be possible to predict the risk of fracture and to explain further some aspects of the mechanics of fracture in the elderly.