Changes in the bones of hyperparathyroid patients were studied with the new technique of total-body neutron activation analysis. Demineralization of the skeleton occurred in most hyperparathyroid patients as has been previously noted. The further finding of no loss of skeletal mass in some patients reinforces a previously formed hypothesis of the existence of two clinically different types of hyperparathyroidism. A low mean P/Ca ratio was observed, possibly the result of an inhibition of proximal tubular resorption corresponding to the increased PTH level. No significant change in the total Ca level was observed in several patients studied at 6-15 months following surgical removal of the parathyroid adenoma. Hypoparathyroid patients exhibited an increase in body Ca in terms of the expected value (Cap). Gains in the total body Ca level in postoperative hypoparathyroidism were modest compared to a marked increase in a patient with idiopathic hypoparathyroidism. The results are consistent with the reduction of bone resorption previously noted in patients with hypoparathyroidism. As demonstrated by the present study, the TBNAA technique provides useful data for research in that it quantitatively links parathyroid function with 2 parameters of skeletal metabolism: total body Ca and P. The data obtained are useful in elucidating the mechanism by which the parathyroids influence skeletal mass and in the evaluation of therapeutic programs.