Ninety-seven male patients with idiopathic calcium urolithiasis and 17 normal male subjects were studied to evaluate the mechanism of idiopathic hypercalciuria with an oral calcium tolerance test, which has been useful in differentiating hypercalciuria. The changes in parathyroid function, such as parathormone and urinary cyclic AMP, and calcium after calcium load differed between absorptive hypercalciuria and renal hypercalciuria. We have confirmed that the change in serum calcitonin after calcium load was also different in these two hypercalciurias. The increase in serum calcium was sufficient to reduce parathyroid function but serum calcitonin was unchanged after calcium load in the control group, in patients with normocalciuria, and those with renal hypercalciuria. Although serum and urinary calcium were more elevated in absorptive hypercalciuria than in the other three groups, parathyroid function was not significantly reduced after loading in absorptive hypercalciuria. In this group only, the serum calcitonin was significantly elevated after calcium load. It is reasonable to suggest that, in this group, because parathyroid function is usually suppressed by intestinal hyperabsorption of calcium, parathyroid function may not be further suppressed by even calcium load. Possibly the significant stimulation of calcitonin may compensate for the lack of suppression of parathyroid function and maintain normal serum calcium levels in absorptive hypercalciuria. These results suggest that the change in serum calcitonin is also useful to differentiate abnormalities of calcium metabolism in patients with hypercalciuria.