In 9 children with idiopathic hypercalciuria, an oral calcium-loading test was performed. After this calcium excretion, vitamin D levels, parathyroid hormone levels and phosphate excretion were measured during a period of calcium restriction, a period of high calcium intake and a period of low calcium intake and phosphate supplementation. In our patients, there was no correlation between calcium excretion following acute and long-term calcium loading. Phosphate excretion was normal during the periods of low and high calcium intake and there were no signs of renal phosphate leakage. Elevated levels of 1,25-dihydroxyvitamin D were found with no significant change after altering phosphate or calcium intake (95% confidence intervals for the difference in 1,25-dihydroxyvitamin D levels were -2.2-15.4 pg/ml in the period with low and high calcium intake; -19.8-28.2 pg/ml in the period with low calcium intake and extra phosphate, and -24.2-19.6 pg/ml in the period with high calcium intake and extra phosphate). These data support the hypothesis of an autonomously elevated 1,25-dihydroxyvitamin D level as pathogenetic mechanism for idiopathic hypercalciuria.