The decrease of bone strength in relation to age and osteoporosis is more pronounced than would be expected from the relative deficit in the amount of bone. Besides bone mass, the mechanical properties of cancellous bone also depend on the microarchitecture and possibly on the molecular structure of inorganic and organic components. The present study examines the bone collagen, especially the collagen cross links, in relation to age and osteoporosis. Samples of vertebral trabecular bone were taken at autopsy from 43 normal individuals, aged 15-90 years. Eleven of these served as sex- and age-matched controls for similar samples from 11 osteoporotic individuals, 70-90 years. The volume of each trabecular bone sample was estimated. After removal of the marrow, the trabeculae were ground to powder and decalcified. The extractability of the bone collagen was studied by repeated extractions with acetic acid and pepsin. The divalent reducible collagen cross-links, dehydro-dihydroxylysinonorleucine (DHLNL) and dehydro-hydroxylysinonorleucine (HLNL), were determined by reducing the bone collagen with tritiated potassium borohydride followed by ion-exchange chromatography. The mature trivalent pyridinium cross links were determined by reverse-phase HPLC with fluorescence detection. The extractability of collagen prepared from the vertebral trabecular bone of control individuals was increased with age. Bone collagen of osteoporotic individuals showed increased extractability and a substantial decrease in the concentration of the divalent reducible collagen cross links (DHLNL reduced by 30% and HLNL by 24%) compared with the sex- and age-matched controls. No alterations were observed in the concentration of the pyridinolines. The divalent reducible cross-links are the most frequent known cross links in bone (2-4 times the concentration of the pyridinium cross links). These changes would therefore be expected to reduce the strength of the bone trabeculae and could explain why the osteoporotic individuals had bone fractures even though the collagen density (mg/cm3) did not differ from that of the sex- and age-matched controls. The microarchitecture of the cancellous bone was not assessed. The osteoporotic and control individuals seemed to have the same amount of trabecular bone, but the quality of the osteoporotic bone collagen was reduced.