In 50 children with hematuria or urolithiasis and idiopathic hypercalciuria, and in 15 control children, urinary calcium/creatinine concentration rates were measured after fasting and after calcium loading. Patients were classified into two groups depending on the results of an orally administered calcium loading test. Children were considered to have absorptive hypercalciuria (42%) when they had low fasting urinary calcium/creatinine concentration ratio (less than 0.21), and a large increase of this index after calcium administration (greater than 0.28). Patients were labeled as renal hypercalciuria (32%) if they had high fasting urinary calcium/creatinine concentration ratio (greater than 0.21), and variable increases of it after calcium overload. A third group of children (26%), were not classifiable by means of this test. Our data support the contention that this simple ambulatory test is very useful in the diagnostic workup of idiopathic hypercalciuria.