After discovering juvenile rheumatoid arthritis (JRA), hematuria, and urolithiasis associated with hypercalciuria in two children, urinary calcium excretion was examined in 38 patients with JRA. Fasting urine calcium/creatinine (mg/mg) (UCa/UCr) ratios were increased (greater than 0.21) in 12 patients, who had a mean UCa/UCr ratio of 0.34 +/- 0.14, compared with 0.09 +/- 0.06 in 26 normocalciuric patients with JRA. Increased UCa/UCr ratios were found more frequently in patients with systemic JRA (P less than 0.05); however, no relationship between UCa/UCr ratios and either functional classification or drug therapy was observed. Four children with increased urine calcium to creatinine ratios were examined more extensively. Twenty-four-hour urine calcium excretion ranged from 4.0 to 7.2 mg/kg/24 hours. An orally administered calcium loading test demonstrated fasting hypercalciuria after dietary calcium restriction in these four patients. Serum calcium, bicarbonate, phosphorus, and parathyroid hormone values were normal. Hematuria was found in six of 12 hypercalciuric patients with JRA but in only three of 26 normocalciuric patients (P less than 0.016). We conclude that urinary calcium excretion is frequently increased in patients with JRA and that hypercalciuria may be related to the pathogenesis of hematuria in some of them.