Abnormalities in renal tubular function have been reported in adult patients with idiopathic renal hypercalciuria. To determine if such abnormalities are present early in the natural history of renal hypercalciuria, we evaluated renal tubular function in ten children with idiopathic renal hypercalciuria, aged 5-17 years. Seven of the children presented with urolithiasis and three with hematuria. Urinary calcium excretion ranged from 4 to 9 mg/kg per day, (5.2 +/- 0.5, mean +/- SEM) with a mean fasting urinary calcium to creatinine ration of 0.31 +/- 0.03. Studies described in this report were performed after 1 week of ingesting a diet containing 1,000 mg calcium, 3,000 mg sodium, and 100 mg purine. Clearance of creatinine ranged from 84 to 159 ml/min per 1.73 m2. Tm phosphate (mg/100 ml GFR) was normal in each child (mean 4.66 +/- 0.06 mg/100 ml GFR). Fractional excretion of uric acid, sodium and beta-2-microglobulin were also normal in each child. Serum bicarbonate concentrations ranged from 21.5 to 27 mEq/l with a mean of 24.4 +/- 0.5 mEq/l and all patients lowered urinary pH to less than 5.5. Hypotonic diuresis demonstrated normal free water clearance with a mean of 12.8 ml/min per 100 ml Cin. Distal sodium delivery and fractional distal sodium reabsorption were normal with a mean of 13.6 +/- 1.2% and 92.7 +/- 0.5%, respectively. Water deprivation studies demonstrated a range of maximum urinary osmolality from 711 to 1,020 mosmol/kg H2O with a mean of 864 +/- 34 mosmol/kg H2O. Seven healthy children, ingesting an identical study diet, concentrated their urine to a mean of 1,059 +/- 31 mosmol/kg h2O.(ABSTRACT TRUNCATED AT 250 WORDS)