Porowski T, et al. - This study was conducted to gauge the usefulness of upper metastable limit osmolality as a new risk index of kidney stone formation in children. It was revealed that the 24-h urine osmolality imparted information regarding the current hydration status. On the other hand, evaporation test determined the urinary potential to crystalize dependent on urine composition. Data indicated that upper metastable limit osmolality could estimate the individual lithogenic capability. It could also detect individuals at risk for stone formation when exposed to dehydration.