Reductions in bone density are a major determinant of vertebral fractures in the elderly population. However, women have a greater incidence of fractures than men, although their spinal bone densities are comparable. Recent observations indicate that women have 20-25% smaller vertebrae than men after accounting for differences in body size. To assess whether elderly women with vertebral fractures have smaller vertebrae than women who do not experience fractures, we reviewed 1,061 computed tomography bone density studies and gathered 32-matched pairs of elderly women, with reduced bone density, whose main difference was absence or presence of vertebral fractures. Detailed measurements of the dimensions of unfractured vertebrae and the moment arm of spinal musculature from T12 to L4 were calculated from computed tomography images in the 32 pairs of women matched for race, age, height, weight, and bone density. The cross-sectional area of unfractured vertebrae was 4.9-11.5% (10.5 +/- 1.4 vs 9.7 +/- 1.5 cm2; P < 0.0001) smaller and the moment arm of spinal musculature was 3.2-7.4% (56.4 +/- 5.1 vs 53.1 +/- 4.4 mm; P < 0.0001) shorter in women with fractures, implying that mechanical stress within intact vertebral bodies for equivalent loads is 5-17% greater in women with fractures compared to women without fractures. Such significant variations are very likely to contribute to vertebral fractures in osteoporotic women.