Age-related fractures are common at skeletal sites with high proportions of cancellous bone such as the hip and spine. Research has shown that measures of bone quantity alone are imperfect predictors of fracture risk, so recent efforts have focused on combining them with measures of bone quality. One aspect of quality that has received little attention is the microstructure of the trabeculae themselves, which are composed of a patchwork of bone structural units (BSU), also known as hemiosteons or trabecular packets, bonded together by cement lines. Any changes in the size of the BSU can be expected to affect the mechanical and failure behavior of cancellous bone. The present work quantified morphometric changes in BSU from the vertebra of 8 young and 8 old individuals as a function of age and 3-D architectural parameters. Reductions in the size of BSU and increases in the proportion of cement line were found to occur with ageing, but these changes were more highly correlated to deteriorating cancellous architecture. These relationships, and the mechanical implications of smaller BSU and increased amounts of brittle cement line, require further investigation.