In ageing men, skeletal fragility is associated with reduced cortical thickness and decreased bone density. To better understand the role of testosterone and 17beta-estradiol regarding these characteristics of skeletal fragility, we correlated their circulating levels with the estimates of mechanical bone properties derived from areal bone mineral density (aBMD) measured by DXA. External diameter and BMD were used to estimate cortical thickness, cross-sectional area (CSA), section modulus, buckling ratio and strength index of the femoral neck and distal radius on 760 men aged 40-85 years. The 17beta-estradiol level was an independent positive determinant of CSA, aBMD and estimated cortical thickness of both bones. In multivariate models adjusted for age, body weight, height, lean body mass and testosterone concentration, men in the lowest quartile of 17beta-estradiol had lower CSA at the femoral neck (4.8%, P<0.001) and distal radius (3.6%, P<0.01) compared with men in the highest quartile. They had also thinner cortical bone at the femoral neck and distal radius (4.8%, P<0.001 and 4.6%, P<0.001, respectively). Furthermore 17beta-estradiol had a negative association with indices of cortical instability (buckling ratio) and a positive association with bending strength (section modulus, strength index) both at femoral neck and radius. Men in the lowest quartile of 17beta-estradiol had higher buckling ratios (femoral neck 4.8%, P<0.002; radius 5.1%, P<0.005), lower strength index (femoral neck 8.5%, P<0.001, radius 6.1%, P<0.01) and greater section modulus at the femoral neck. However, there were no between-quartile differences in external diameter in any bone sites. Similar, even though somewhat smaller, between-quartile differences were found for bioavailable 17beta-estradiol. Neither total testosterone nor apparent free testosterone concentration was associated with any bone variables after adjusting for age, body weight, body height, and lean body mass and 17beta-estradiol level. In conclusion, in elderly men, low concentration of 17beta-estradiol (total and bioavailable) was associated with a decreased cortical thickness and with a deterioration of biomechanical parameters of long bones (lower section modulus and strength index, higher buckling ratio).