The pathogenetic roles of idiopathic renal hypercalciuria and absorptive hypercalciuria in children with urolithiasis have not yet been determined. Oral calcium loading studies were performed in 21 children with unexplained calcareous urolithiasis. Thirteen children, aged 20 months to 17 years, were found to have renal hypercalciuria after an overnight fast (urinary calcium-urinary creatinine [UCa/UCr] ratio in milligrams, greater than 0.21). Four children were found to have absorptive hypercalciuria. In this group, fasting UCa/UCr values were normal (SEM, 0.12 +/- 0.02); however, UCa/UCr values were elevated (SEM, 0.31 +/- 0.01) after the oral calcium load. Serum parathyroid hormone values were normal in all children with hypercalciuria. Urinary calcium excretion was normal in four patients. These data indicate that hypercalciuria may frequently occur in children with urolithiasis and that detailed metabolic evaluation is warranted in children with kidney stone disease.