Age-related changes in the microanatomic structure of human iliac crest trabecular bone have been investigated in biopsies from 90 subjects, using a new computerised method which automatically identifies and quantifies nodes, free ends and a number of topologically defined struts. In both sexes there was a significant age-related decrease in the number of nodes and of trabecular struts, corrected for section area. In females, there was also a significant age-related decrease in the node to free end ratio (P < 0.001), and the combined node-to-node and node-to-loop strut length, expressed as a percentage of total strut length (P < 0.001) and a significant increase in free end to free end strut length (P < 0.001). In males, the only additional age-related change was a significant increase in the cortex to free end strut length (P < 0.005). These results indicate that loss of trabeculae resulting in decreased interconnectedness of the normal trabecular bone structural pattern, plays an important role in agerelated bone loss in females. Removal of trabeculae also occurs in males but is less prominent, implying that trabecular thinning makes a greater contribution to age-related bone loss in males.