In previous studies, we observed increases in the circulating concentration and production rate of 1,25-dihydroxyvitamin D (1,25-(OH)2D) in a large majority of patients with the syndrome of absorptive hypercalciuria. In the present study, the hypothesis that 1,25-(OD)2D production might be relatively autonomous in this syndrome was tested by fashioning a suppression test in which patients were challenged with a short-term increase in dietary calcium intake. We found that contrary to our hypothesis, the circulating concentration of 1,25-(OH)2D was remarkably sensitive to calcium intake in 15 patients with absorptive hypercalciuria (mean decrease, from 74 to 49 pg per milliliter, P less than 0.001). When this challenge was prolonged for two weeks, however, patients with absorptive hypercalciuria had evidence of an apparent "escape" phenomenon, in which the circulating concentration of 1,25-(OH)2D rebounded toward its initial level and the renal tubular phosphate threshold fell markedly. These findings provide evidence for disordered control of renal phosphate handling and 1,25-(OH)2D production in absorptive hypercalciuria and suggest a linked rather than a cause-and-effect relation between these two abnormalities.