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Objective: In this study, we measured urinary osteopontin (OPN) concentrations in urolithiasis patients as well as in healthy volunteers, and investigated the relationship between urinary excretion of OPN and urinary supersaturation level., Methods: Supersaturation levels (AP indexes) were determined by using Tiselius's index. Crystals with a maximum diameter of 12 ìm or larger and less than 5 ìm were counted by scanning electron microscopy. A sum of cross-sectional areas of crystals was also calculated as the total crystal volume (VT)., Results: Urinary OPN concentrations in the group with no urinary stone were significantly higher than that in the urolithiasis patients with a tendency toward stone enlargement. AP indexes were observed to be significantly higher in patients with stone enlargement, whereas urinary OPN concentrations bore no definite relation to the urinary supersaturation levels. VT and number of large crystals (12 ìm or larger) in patients with a tendency toward stone enlargement were higher than healthy volunteers, but no differences were found between the number of micro-crystal with the diameter of less than 5 ìm. On the basis of the plots of VT and OPN concentrations, regression analysis revealed that VT and log OPN had a significant correlation., Conclusion: Urinary OPN tended to be lower in cases with larger crystal volumes and is potentially associated with crystal growth for inhibitory effect.