The material properties of cancellous bone from patients with osteoporosis (OP) or osteoarthritis (OA) were determined and compared with normal controls. Samples were selected from defined sites in human femoral heads which are subjected to different loads in vivo. Overall, OP bone had the lowest stiffness and OA the highest, and this same order was reflected in the apparent densities of the bone, with OA being the most dense and OP the least. Normal and OP bone were found to have very similar stiffness–density relationships and composition. However, OA bone differed significantly from normal. The stiffness of OA bone increased more slowly with apparent density and its material density was significantly reduced. These findings were due to an altered composition of the bone in which the mass fraction of mineral is 12% less than normal. There was also greater site variation of both apparent and material density, suggesting an altered sensitivity to applied load. These results support the concept that osteoporosis is a loss of normal bone. They also provide evidence for the hypothesis that osteoarthritis is, at least partly, a bone disease in which proliferation of defective bone results in an increase in bone stiffness.