Intestinal oxalate absorption is an important part of oxalate metabolism influencing its urinary excretion and its measurement can be a valuable diagnostic tool in hyperoxaluric disorders. In this study, we use [(13)C(2)]oxalate absorption under standardized dietary conditions to assess intestinal oxalate absorption and its impact on urinary oxalate excretion. Tests were conducted in age-matched pediatric patients that included 60 with idiopathic calcium oxalate urolithiasis, 13 with primary hyperoxaluria, and 35 healthy children. In the idiopathic stone formers, median oxalate absorption was significantly higher than that in the controls or in patients with primary disease. From standardized values obtained in control patients, oxalate hyperabsorption was detected in 23 patients with idiopathic disease but not in any patients with primary hyperoxaluria; therefore, a significant correlation between intestinal absorption and urinary excretion was found only in those with the idiopathic disease. We have shown that increased intestinal oxalate absorption is an important risk factor of idiopathic calcium oxalate urolithiasis. In contrast, low intestinal oxalate absorption in patients with primary hyperoxaluria indicates that only foods with excessive oxalate content be restricted from their diet.