Twenty-three children with idiopathic hypercalciuria and 7 control children were studied. Patients were classified into three groups according to the response to an oral calcium load. Five children displayed absorptive hypercalciuria (UCa/Cr 0.14 +/- 0.04 mg/mg with poor calcium diet and 0.33 +/- 0.16 after loading, p less than 0.01); 10 were classified renal hypercalciuria (UCa/Cr during the fasting state of 0.28 +/- 0.09 and 0.31 +/- 0.11 after loading) and 8 were classified as alimentary hypercalciuria (UCa/Cr 0.14 +/- 0.07 during the fasting state and 0.15 +/- 0.04 after loading). The urinary cAMP showed no significant differences in any of the three groups compared to the control. Magnesiuria did not show significant differences between the two forms of hypercalciuria and the control; a significant increase was observed after the loading and a positive correlation between magnesiuria and calciuria in the absorptive form. Our study supports the validity of the test for the classification of the different forms of hypercalciuria in children. The urinary cAMP is not a valid approach for classification. The difference observed in the magnesiuria suggests a different pathogenic mechanism between the two types of idiopathic hypercalciuria.