Osteoporotic compression fractures of the spine differ from most other age-related fractures in that they usually are associated with minimal trauma and with loads no greater than those encountered during normal activities of daily living. With aging and osteoporosis, there is progressive resorption of bone, resulting in reductions in bone density, thinning of trabeculae, and loss of trabecular contiguity. These changes in trabecular bone structure are associated with losses in bone strength which are disproportionate to the reductions in bone mass alone. To explain this disproportionate loss of bone strength, the prevailing opinion is that density reductions in the vertebral centrum are accompanied by a reduction in the number of trabeculae, by preferential resorption of horizontal trabeculae, and by hypertrophy of the remaining vertical trabeculae. To evaluate this view of vertebral morphology, we performed threedimensional stereological analysis of trabecular bone extracted from midsagittal sections of first lumbar vertebral bodies from 12 donors spanning an age of 27-81 years. We found that both the number (R² = 0.63, P < 0.01) and thickness (R² = 0.91, P < 0.01) of trabeculae decreased linearly with density (as expressed by bone volume fraction) whereas the spacing between the trabeculae (R² = 0.61, P < 0.01) increased reciprocally. There were more vertical trabeculae with transverse trabeculae at all densities, and the number of vertical trabeculae changed with density at twice the rate of the number of transverse trabeculae (P < 0.001). These data do not support the prevailing view that there is preferential resorption of horizontal trabeculae or hypertrophy of the remaining vertical trabeculae. Bone density was also a strong (R² = 0.90, P < 0.01) power law function of the ratio of trabecular thickness to mean intertrabecular spacing. From buckling theory, the critical buckling load of a trabecula is related to this ratio of trabecular thickness to effective length. The changes in trabecular morphology observed with decreasing bone density thus pose a "triple threat" to the strength and stability of vertebral trabecular bone, as not only are there fewer trabeculae, but the remaining trabeculae are both thinner and longer.