In kidney stone disease, or nephrolithiasis, stones (calculi) are present in the urinary tract. A kidney stone is a solid piece of material that forms in the urine when normal urine substances become highly concentrated.  These normal substances include calcium, oxalate, magnesium and phosphorus. 

Kidney stones may grow in quick spurts or very slowly over time. They can cause sudden and severe pain. Any child who has had a kidney stone is at increased risk of developing another stone in the future.