Brushite (CaHPO(4).2H(2)O) was considered to govern the formation of renal calculus of calcium phosphate origin. The degree of saturation of urine with respect to this phase was therefore calculated. This value was obtained from the ratio of the activity product of Ca(++) and HPO(4) (m) (K(sp)) before and after incubation of urine with brushite. The errors in the calculation of K(sp) were largely eliminated by this procedure.The urine of patients with idiopathic hypercalciuria and recurrent calcium-containing renal calculi was supersaturated with respect to brushit largely because of the high urinary concentration of Ca(++). The urine of normocalciuric subjects was undersaturated except at high urinary pH. This technique of estimating the degree of saturation of urine should allow a quantitative assessment of the various therapeutic regimens recommended for patients with nephrolithiasis.