Bone densitometry using I125 is a very easy method of investigation; it may be used repeatedly and simultaneously at different bone levels without distress to patients. The problems involved were those of the value, semeiological significance and possible limitations of this method in the investigation of uraemic osteopathy. These problems were solved successfully in tests carried out on 87 patients of both sexes suffering from various degrees of chronic nephropathy and on chronic haemodialysis patients. The method is reliable: reproducibility tests in normal and uraemic subjects were satisfactory; the indexes remain constant, usually in normal subjects. The parameters are "Bone Mineral Content" and "Bone Index". Trabecular bone is more sensitive than cortical bone, although comparisons between the two types of bone produce interesting data. There are difference dependent on sex. The method which is more sensitive than others enables: 1) Normal and nephropathic subjects, to be distinguished from one another. The latter generally show a lower mineral content and bone index (clinical picture of osteomalacia, osteoporosis, hyperparathyroidism) seldom a higher or a normal one. 2) The same bone sites to be examined over a given period. 3) Treatment of patients to be monitored.