Post-mortem specimens of bone from the second metacarpal, third phalanx, iliac crest, skull, vertebral bodies and ribs were obtained from 76 hospital patients. Four cases of osteomalacia were found with low ash/residue ratios and widened osteoid borders. The remaining 72 cases showed varying degrees of osteoporosis. The ash/cm3 of anatomical bone varied between 137 mg and 481 mg, and the ash/mm of shaft had values between 13–4 mg and 63 mg. Marked histological variations occurred in the bones from different sites and this was probably related to the differing proportions of trabecular bone, which has a higher turnover rate than cortical bone. In the ribs the degree of rarefaction correlated with the number and size of osteocytic lacunae, and bone resorption appeared to be due mainly to osteolysis. The changes were prominent on the endosteal and Haversian surfaces and occurred to a lesser extent on the periosteal surfaces. Areas of necrosis were common. In the skull and iliac crest rarefaction was also related to the degree of osteocytic activity. In the vertebral bodies changes were more severe than in the other bones and osteoclasis played a more significant role. There were sites where the outer bony shell had been destroyed, so that the marrow was in contact with the intervertebral disc or periosteum. On the basis of histology and ash content there was no clear distinction between physiological and pathological osteoporosis; loss of bone appeared to be a continuing process. These and other findings are consistent with the view that people with densely calcified bones in early life will have some protection against developing pathological osteoporosis when, in later life, part of die mineral is lost.