Reference values for the urinary calcium/creatinine ratio (Ca/Cr ratio) in the first morning urine were established in 361 healthy children aged 5 to 15 years, on unrestricted diets. The urinary Ca/Cr ratio in the urine upon arising was independent of sex but dependent upon age. The measurement of the urinary Ca/Cr ratio in the urine upon arising while on unrestricted diets may be a reasonable screening test for elevated calcium excretion. On the basis of the urinary Ca/Cr ratios in the urine upon arising during unrestricted diets and the calciuric response to calcium restricted diets and the oral calcium loading test, idiopathic hypercalciuria (IH) was subclassified into three groups: (1) absorptive hypercalciuria; (2) renal hypercalciuria; (3) dietary hypercalciuria. The pathogenesis of IH is controversial. Our data suggest that disordered 1,25 (OH)2 vitamin D metabolism with excessive urinary phosphate excretion occurs in absorptive hypercalciuria.