To investigate the effects of age on rates of bone loss and the relationship between amount of trabecular bone and clinical severity of osteoporosis, trabecular mineral density of the lumbar spine (VMD) was measured in 55 osteoporotic women and 133 healthy women with both single energy (SE) and dual energy (DE) quantitative computed tomography (QCT). The amount of marrow fat was indirectly estimated by the difference (Δ) between DE and SE VMD values. The rate of bone loss in the normal women was 1.14%/yr with SE and 1.03%/yr with DE QCT. Osteoporotic patients had a VMD decline of 1.62%/yr with SE and 1.17%/yr with DE QCT. Osteoporotic patients had significantly lower (P < 0.0001) mean SE and DE VMD at any age, but VMD was not significantly different among groups characterized by different number of fractures or different radiographic severity of fractures. The threshold values of VMD below which the risk of having fractures was increased were 99.8 and 118.7 mg/cm3, respectively, for SE and DE QCT. Dispersion around the mean, overlap between osteoporotic and healthy women, and the incidence of asymptomatic osteoporosis were greater with DE than SE QCT. Osteoporotic women had higher Δ values (P < 0.05) compared to normal women, but Δ did not correlate with clinical severity of osteoporosis. The results indicate that 1) factors in addition to the amount of spinal trabecular bone determine the number and severity of fractures in osteoporotic women; 2) DE QCT reduce the VMD underestimation due to intravertebral fat content, but not the overlap between osteoporotic and normal women; and 3) further anatomical studies of osteoporotic vertebrae are necessary to investigate the effect of age on intravertebral fat.