Abstract only Formation of kidney stone affects approximately 1 in 11 people in the U.S., and it can develop into various adverse renal outcomes. Although American Urological Association guideline recommends having at least 2.5 L of urine daily by having a high fluid intake to reduce 50% of recurrence rate in stone formers, few studies have evaluated the association between urine concentration as a marker of hydration status and the risk of kidney stone formation. PURPOSE To identify the threshold of optimal hydration in order to classify patients as high risk for kidney stone formation. METHODS Total of 302 subjects from Southern France with a history of recurrent kidney stones were retrospectively included in the study (female: 40.4 %, age: 46±14 y, BMI: 25.3±5.0 kg·m −2 ). Tiselius calcium oxalate crystallization risk index (CRI) was calculated based on urinary calcium (6.42±3.60 mmol·24h −1 ), oxalate (0.36±0.14 mmol·24h −1 ), citrate (32.0±17.5 mmol·24h −1 ), and magnesium (4.31±1.74 mmol·24h −1 ) from 24 h samples. The ability of 24 h urine osmolality to classify patients as high risk for kidney stone crystallization was examined through the receiver operating characteristic (ROC) curve analysis. High risk for kidney stones crystallization was defined as CRI >1.61 and >1.18, for males and females, respectively. The correlation coefficient was also examined between 24 h urine osmolality (538±198 mOsm·kg −1 ) and 24 h urine volume (1,948±823 mL·24h −1 ) and 24 h CRI (1.23±0.71). RESULTS The overall accuracy for diagnosing high risk of stone formation (AUC, area under the curve) was 84.9%, with threshold of 577 mmol·kg −1 (sensitivity: 78.4%, specificity: 81.6%) for the entire sample. When data where analyzed by gender, AUC for females was 84.6%, with cut‐off point of 501 mmol·kg −1 (sensitivity: 83.3%, specificity: 76.0%). Males had AUC of 85.8% with thresholf of 577 mmol·kg −1 (sensitivity: 85.5%, specificity: 77.6%). A moderate negative association was found between urine volume and urine osmolality ( r =−0.632, P <0.05). Also a moderate but positive association was found between 24 h urine osmolality and Tiselius CRI index ( r =0.638, P <0.05). CONCLUSION Urine osmolality >501 and >577 mmol·kg −1 was assosicated with greater risk for kidney stone formation in patiens with history of recurrent kidney stone.