To evaluate the adequacy of simple calcium restriction for patients with idiopathic calcium stones the effect of 5 days of calcium restriction without oxalate restriction on renal excretion of calcium and oxalate, and the corresponding probability of stones were assessed in 50 controls and 48 patients. Renal excretion of calcium decreased and that of oxalate increased significantly in all groups but the importance of the changes was critically dependent upon the underlying pathophysiological condition. The probability of stones decreased only in patients with absorptive hypercalciuria type II owing to the usual excessive calcium intake and increased in those with absorptive hypercalciuria type I and renal hypercalciuria, which are associated with true hyperabsorption of calcium and represent the 2 forms of idiopathic hypercalciuria. We believe that simple calcium restriction is beneficial for patients with idiopathic calculi only when the hypercalciuria is caused by exaggerated intake of calcium, since it increases the probability of stones in patients with idiopathic hypercalciuria. Calcium restriction always is associated with an increase in oxalate excretion, suggesting that simultaneous oxalate restriction should be added in all cases to decrease the probability of stones.

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