Project Abstract The prevalence of urinary stone disease (USD) has risen over the last few decades and remains a worldwide problem. Twenty-four urine testing plays an important role in guiding specific dietary and pharmacologic approaches for secondary prevention of kidney stones. Indeed, the American Urological Association (AUA) and the European Association of Urology (EAU) both recommend 24-hour urine testing in evaluating and treating high risk individuals with kidney stones. However, scant evidence exists for whether incorporation of 24-hour urine testing in the care of high risk stone formers leads to sustained changes in diet, medication use or a decrease in stone recurrence. To address this gap in knowledge, we have identified a cohort of patients with USD in the Veterans Health Administration (VHA) from 2005 – 2014. Preliminary data indicates a prevalence of about 6.5-8.7% for 24-hour urine testing in this population, which is similar to numbers seen in a recent study performed in a different database [1]. We propose to determine the effectiveness of 24-hour urine testing in modifying stone risk and preventing stone-related ER visits or urologic procedures in stone formers in the Veterans Affairs (VA) population. We hypothesize that 24-hour urine testing will be associated with increased utilization of stone prevention medications as well as an improvement in urine parameters measured on subsequent 24-hour testing. We also hypothesize that 24-hour urine testing, after adjustment for confounding by indication, leads to a decrease in stone risk and USD-related ER visits or urologic procedures. Finally, we will use mass spectrometry techniques to identify novel metabolites in the urine of calcium stone formers to predict risk for recurrent stones. This information has the potential to identify novel solutes that may protect individuals with USD from stone recurrence by alerting clinicians to track urine excretion of additional anions that may bind calcium and keep it soluble in the urine.