A method of computed tomography (CT) image analysis of lumbar vertebrae has been developed, providing a visualization of the trabecular network as it is represented in a 1.5 mm-thick CT image. We measured the length of the network and the number of discontinuities found in the image. The ratio of these measurements was called the "trabecular fragmentation index" (TFI). CT images from 71 women between the ages of 50 and 59, and 94 women between the ages of 60 and 69 were divided into three groups according to quantitative computed tomography (QCT) vertebral density and to the presence or absence of crushing and fractures. The measure of the network length versus the vertebral area was significantly higher in normal subjects than in osteoporotics. A TFI threshold at 0.195 could separate the normal subjects, regardless of the decade, from osteoporotic ones. In females between 50 and 69 years of age, TFI was 0.166 (SD = 0.031) for the normal group and 0.248 (SD = 0.082) for osteoporotics. The osteopenic group without fractures but low bone mineral density (BMD) showed an intermediate TFI of 0.195 (SD = 0.05), placing this population on both sides of the threshold. Correlation between TFI and BMD was only -0.60. TFI could provide new information in vivo about the state of trabecular structure, particularly in the osteopenic group.