A computerized program was used to calculate the activities of the calcium, phosphate, and hydrogen phosphate ions. The most important determinants for ion-activity products of calcium phosphate and brushite were calcium, phosphate, citrate, urine volume, and pH. These urine variables were used to derive indices corresponding to ion-activity products of calcium phosphate (AP[CaP] index) and calcium hydrogen phosphate (AP[Bru] index). Factors were given to adapt these indices to collection periods shorter than 24 h. Relationships between the AP(CaP) index and ion-activity products of hydroxyapatite and octacalcium phosphate were also formulated. With urine electrolytes within the normal range a very good correlation was obtained between these indices and corresponding ion-activity products. The coefficient of correlation was better than 0.99. The presented indices might be useful in the evaluation of patients with renal calcium stone formation.