Abstract In vivo neutron activation analysis and whole-body counting techniques were used to measure the body composition of patients with chronic renal failure. Levels of total body calcium, phosphorus, sodium, chlorine, nitrogen, and potassium were measured in uremic patients at various times prior to and during maintenance dialysis. The metabolic derangements in individuals with chronic renal failure often result in the development of overt secondary hyperparathyroidism. The action of PTH on bone is associated with osteodystrophy as well as with extensive metastatic calcification. These effects present major problems in the therapeutic management of uremic patients. The most interesting finding is that of normal total body calcium (Ca/height) measured in uremic patients both prior to and during hemodialysis. Presumably, this high total Ca, occurring simultaneously with a condition such as osteoporosis, reflects the translocation of Ca from skeletal tissue to soft-tissue depots. Uremic patients on dialysis usually experience a loss of total body Ca. TBNAA (total body neutron activation analysis) alone does not distinguish between Ca in bone and Ca in the soft tissue; hence data from tracer studies are required for this analysis. However, following parathyroidectomy in uremic patients with extensive metastatic Ca, TBNAA measurements show a decrease in total body Ca concurrent with reduction of metastatic calcification as indicated by roentgenographic evidence. With renal failure, phosphate is released from the skeleton, and hyperphosphatemia ensues. The P/Ca values for patients on hemodialysis are, on the average, slightly higher than those of normal persons and of uremic patients not on dialysis. The total body sodium in uremic patients was, on the average, higher than that observed in a normal contrast population, as well as higher than that of a group of osteoporotic patients. However, no causal relationship was established between the reduction of total body Na during dialysis and the observed improvement in the hypertensive condition. There was little evidence of K depletion in uremic patients maintained on chronic dialysis as determined by whole-body counting. With few exceptions, uremic patients were found to have body K levels grams of K per kilogram of body weight) normal for their age and sex. The mean total-body nitrogen levels of the uremic patients expressed in terms of body K were similar to those of the normal contrast population, but lower than that of osteoporotic patients. The new insights acquired by the application of in vivo neutron activation technique in studying chronic renal disease, particularly with patients on hemodialysis, are to date chiefly suggestive. However, the acquisition of further data appears to be of considerable value for the understanding of chronic renal failure and the managment of patients through hemodialysis, particularly with respect to associated osteodystrophic changes.