An idealized, structural model of vertebral trabecular bone is presented. The architecture of the model (thick vertical columns and thinner horizontal struts) is based on studies of samples taken from the central part of vertebral bodies from normal individuals aged 30 to 90 years. With trabecular diameters and spacings typical for persons aged 40, 60, and 80 years respectively, the model accounts reasonably well for age-related changes in vertical and horizontal stiffness and trabecular bone volume, as seen in experimental data. By introducing a measure for the randomness of lattice joint positions in the modeled trabecular network, it is demonstrated that the apparent stiffness varies by a factor of between 5 and 10 from a perfect cubic lattice to a network of maximal irregularity, even though trabecular bone volume remains almost constant. A considerable change in mechanical behaviour is also seen, without changing the overall trabecular bone volume, when the bone material is slightly redistributed among vertical and horizontal trabeculae. It is concluded that measured bone mass should not be the sole indicator of trabecular bone biomechanical competence (stiffness and stress). It is crucial that measurements of bone density are considered in combination with a detailed description of the architecture.