The hypothesis of this study is that the in situ thermostability of collagen in bone changes with age and such changes relate to the structural properties of collagen and/or the interaction between the collagen and mineral phases. To test the hypothesis, the effect of age on the in situ thermostability of collagen in human bone and its correlation with the mineral and collagen phases were investigated. In this study, 30 human cadaveric femurs were collected and divided into three age groups: young adults (20-45 years), middle aged (46-69 years), and elderly (over 70 years of age). The in situ thermostability of collagen was assessed using high performance liquid chromatography (HPLC) in terms of the amount of heat-induced collagen denaturation at eight temperatures between 25 degrees C and 140 degrees C. In addition to the density and weight fraction of the mineral and organic phases, the concentration of collagen crosslinks was measured to assess the structural integrity of collagen. The results of this study indicate that the in situ thermostability of collagen increases with increasing age, and such age-related changes correlate with the following: collagen molecular structure, amount of noncalcified collagen, and the fraction and density of the mineral phase in bone. These results suggest that the age-related changes in the in situ thermostability of collagen most likely relate to the collagen structure and its interaction with the mineral phase. In addition, the bone remodeling process may play a role in the age-related changes in collagen thermostability because noncalcified collagen is commonly associated with this process.