Twelve children (5 males and 7 females, mean age 8.5 years) with idiopathic hypercalciuria (IH), 9 with absorptive IH (1 type I and 8 type II) and 3 with renal IH, were followed in our outpatient hospital from September 1981 to March 1987. Mean (+/- 1 SD) free diet calciuria was 5.9 +/- 1.66 mg/kg/day. Diagnosis was made measuring urinary Ca/Cr ratio after controlled diet and calcium loading test. Serum parathormone (PTH), 1,25-(OH)2-D3 and calcitonin (TCT) values and renal threshold phosphate concentration (TmPO4/GFR) were also studied. Serum PTH was normal in all children. Serum TCT levels were significantly increased in the patients with IH compared with controls (p less than 0.001) while serum 1,25-(OH)2-D3 levels were significantly reduced compared with controls (p less than 0.001).