An ultrasonic technique and microtensile testing were used to determine the Young's modulus of individual trabeculae and micro-specimens of cortical bone cut to similar size as individual trabeculae. The average trabecular Young's modulus measured ultrasonically and mechanically was 14.8 GPa (S.D. 1.4) and 10.4 (S.D. 3.5) and the average Young's modulus of microspecimens of cortical bone measured ultrasonically and mechanically was 20.7 GPa (S.D. 1.9) and 18.6 GPa (S.D. 3.5). With either testing technique the mean trabecular Young's modulus was found to be significantly less than that of cortical bone (p < 0.0001). However, the specimens were dried before microtensile testing so Young's modulus values may have been greater than those of trabeculae in vivo. Using Young's modulus measurements obtained from 450 cubes of cancellous bone and 256 cubes of cortical bone, Wolff's hypothesis that cortical bone is simply dense cancellous bone was tested. A multiple regression analysis that controlled for group membership showed that Young's modulus of cortical bone cannot be extrapolated from the Young's modulus vs density relationship for cancellous bone, yet the Young's modulus of trabeculae can be predicted by extrapolation from the relationship between Young's modulus vs density of the cancellous bone. These results suggest that when considered mechanically, cortical and trabecular bone are not the same material.