The effect of long-term thiazide therapy on the intestinal Ca absorption was measured in 10 well-defined cases of absorptive hypercalciuria with intestinal hyperabsorption of Ca and 8 with renal hypercalciuria ("renal leak" of Ca), many of whom had hyperabsorption of Ca. In most cases of absorptive hypercalciuria, the intestinal hyperabsorption of Ca persisted during treatment, despite restoration of normal urinary Ca. In contrast, the intestinal Ca absorption decreased significantly during thiazide therapy in 7 of 8 patients with renal hypercalciuria commensurate with the "correction" of the renal leak of Ca and secondary hyperparathyroidism. The results support the hypothesis that the intestinal hyperabsorption of Ca in absorptive hypercalciuria may be primary, whereas that in renal hypercalciuria may be associated with the hyperparathyroid state.