The amounts of bone resorbed and formed in each bone remodelling unit are important determinants of bone mass. The mean wall thickness (MWT), which indicates the amount formed, decreases with age;​ less is known about changes in resorption depth although two studies have reported some evidence in favor of an age-related decrease. We have calculated mean interstitial bone thickness (MIBT), which is indirectly related to resorption depth, from measurements of MWT and mean trabecular plate thickness (MTPT) in 48 normal subjects, aged 19–80 years. No significant agerelated changes in MIBT were found in either sex, or in both sexes grouped together, whereas, MWT and MTPT showed a significant decrease with age (P<0.001 and <0.05 respectively). Detailed analysis of possible relationships between MIBT and resorption depth revealed that, assuming formation never exceeds resorption, a decreased MIBT may be associated with an increased, constant, or decreased resorption depth;​ an increased MIBT can only result from a decreased resorption depth. A constant MIBT may indicate either an unchanged or decreased resorption depth;​ in the former case, MTPT is unchanged whereas a decreased resorption depth is associated with a decreased MTPT. In view of the age-related reduction in MTPT, our results are therefore consistent with earlier reports of an age-related decrease in resorption depth. No evidence for an increased resorption depth during the menopause was found in this study.