Using peripheral quantitative computed tomography (pQCT) we assessed trabecular and cortical bone density, mass and geometric distribution at the tibia level in 512 men and 693 women, age range 20-102 years, randomly selected from the population living in the Chianti area, Tuscany, Italy. Total, trabecular and cortical bone density decreased linearly with age ( p<0.0001 in both sexes), and the slope of age-associated decline was steeper in women than in men. In men, the cortical bone area was similar in different age groups, while in women older than 60 years it was significantly smaller by approximately 1% per year. The total cross-sectional area of the bone became progressively wider with age, but the magnitude of the age-associated increment was significantly higher in men than in women ( p<0.001). The minimum moment of inertia, an index of mechanical resistance to bending, remained stable with age in men, while it was significantly lower in older compared with younger women (0.5% per year). The increase in bone cross-sectional area in aging men may contribute to the maintenance of adequate bone mechanical competence in the face of declining bone density. In women this compensatory mechanism appears to be less efficient and, accordingly, the bone mechanical competence declines with age. The geometric adaptation of increasing cross-sectional bone size is an important component in the assessment of bone mechanical resistance which is completely overlooked, and potentially misinterpreted, by traditional planar densitometry.