The structural and geometrical properties of the tibia shaft were investigated at two sections by means of computerized tomography (CT) in 78-year-old women with high (n = 19) and low (n = 17) calcaneal bone mineral density (BMD, g/cm3) previously measured by 125I-photon absorption. The high BMD group had a 20-21% higher tibial BMD and 9-12% higher bone cross-sectional area than was observed in the low BMD group. The distribution of bone mass indicated that the low BMD group had lost bone mainly from the endosteal surface, especially in the anterior part of the tibia. However, both groups had a similar basic pattern of mass distribution at the measured sections. The high BMD group had higher moments of inertia at the upper section than the low BMD group. The differences between the groups were more pronounced when only the high density areas were included. At the lower section, the differences between the groups also appeared significant at the high density levels. There were no group differences in the area moments of inertia. The results suggest that the true distribution of bone mass should be taken into account in determining the moments of inertia. In the tibia, determination of the cross-sectional mass distribution of bone combined with BMD should have a better discriminatory capability than BMD only in studying bone strength and fracture risk.