A family of idiopathic hypercalciuria (IH), 3 symptomatic and 2 asymptomatic, plus 3 normal subjects were given the 1 Gm oral calcium challenge. Biochemical parameters measured included: serum and urinary calcium and phosphate, urinary cyclic AMP, and serum intact and carboxyl-terminal parathyroid hormone. Major differences between the normal control and the family with IH include: (1) higher calcemic response in the family with IH (0.9 vs 0.4 mg/dl); (2) a fall in carboxyl-terminal PTH and urinary cyclic AMP in the IH family in contrast to control subjects in whom there were no changes; (3) a rise in serum phosphorus in the IH family (0.8 vs 0.2 mg/dl, p less than 0.05). Urinary excretion of calcium, phosphorus, and sodium after the calcium challenge was minimal. The oral calcium challenge is a simple and useful test in demonstrating increased calcium absorption even in asymptomatic relatives of patients with idiopathic hypercalciuria.