No detailed biochemical analysis has been carried out of the compositional changes in the collagen matrix of avian bone in relation to increased bone fragility in osteoporosis. We have shown that osteoporosis in avian bone is certainly not just a simple loss of apatite and collagen, but involves significant changes in the biochemistry of the collagen molecule and consequently in the physical properties of the fibre. The decreased mechanical strength and the change in the thermal stability can be directly related to changes in post-translational modifications, i.e. lysine hydroxylation and the intermolecular cross-link profile. The increased hydroxylation and change in cross-linking are consistent with increased turnover of the collagen, possibly in an attempt to initiate a repair mechanism which, in fact, leads to an acceleration in the increase in fragility of the bone. Clearly there are post-translational modifications of the newly synthesized collagen in avian osteoporosis, and these changes may play a role in the pathogenesis of the disease.