Ionic calcium, calcium binding sites, and other urinary variables were measured in 58 patients with idiopathic calcium nephrolithiasis and 36 normal subjects. The patients showed higher urinary concentrations of calcium. The mean calcium excretion (mmole/24 hr) was 4.45 +/- 0.56 (+/- 1 SEM) in patients and 2.19 +/- 0.22 (+/- 1 SEM) in normal subjects. This difference was highly significant (P less than 0.001). The mean ionic calcium excretion (mmole/24 hr) was 1.90 +/- 0.21 (+/- 1 SEM) for patients and 0.97 +/- 0.12 (+/- 1 SEM) for control subjects. The normal subjects showed significantly higher (P less than 0.01) concentrations and total excretions of magnesium and citrate. Excretory patterns for sodium, potassium, phosphate, and oxalate were not significantly different. The normal subjects had higher mean urinary concentrations of binding sites for calcium ions (23.2 +/- 4.8 mM) than the patients (18.5 +/- 2.9 mM). However, as the patients had higher urinary volumes the difference in the 24-hr excretion of calcium binding sites was not significant statistically. Out of 58 patients 43 (74%) were hypercalciuric. Twenty patients (46%) were categorized as an absorptive group and one patient as a resorptive type, and for the rest of the patients (51%) the mechanism of hypercalciuria remained unidentified. Only two of the control subjects (5%) were found to be hypercalciuric under calcium restricted diet conditions. Though these "control" subjects excreted a high amount of calcium there was no associated increase in the fraction of the calcium in the ionic form (0.37). Patients, however, still had relatively high fractions of ionic calcium (0.48 +/- 0.03).