Changes in calcium concentration were induced by an infusion of disodium-EDTA or calcium in 2 members of a family suffering from hypocalciuric hypercalcaemia ( FHH ) associated with interstitial lung disease. These changes in calcium demonstrated an inverse linear relationship with the changes in serum parathyroid hormone (PTH). Infusion of EDTA in control subjects and in patients with an adenoma or hyperplasia of the parathyroid glands also showed inverse relationships between calcium and PTH. The correlation between serum calcium and serum PTH was significant over the range observed during the induced hypo- and/or hypercalcaemia in controls and in patients with FHH or adenoma. The regressions were, however, shifted relative to each other: in comparison with controls, the FHH was displaced upwards and to the right, although not as far as the adenomas. These findings suggest the existence of an elevated set point for extracellular calcium (or calciostat ) in FHH .