Introduction 1 2 2 1 3 5 6 CaOx CaOx 7 8 2+ 2− 9 2+ 2− 9 10 9 10 9 Materials and methods 11 9 Bonn Risk Index 12 2+ 2− P P Results 1 13 Fig. 1 Body mass index (BMI) in studied girls and boys aged 3–18 years compared to age and sex-matched reference range  2 2+ 2− 2+ 2− 2+ 2− Fig. 2  x-axis 2−  y-axis  2+ 2− 2+ 2− 2+ 2− 2+ 2− 2+ 2− 2+ 2−  3 Fig. 3 left box  3 The crystallization values of CaOx, based on BRI in healthy children aged 3–18, did not exceed 2.66 1/L. No significant differences were found in the BRI values between boys and girls in either age group. 1 2 2 2 2 2 R P 2 R P 2 Table 1 Bonn Risk Index values in children aged 3–18 years in relation to body surface area and body weight   Median Minimum Maximum 5th percentile 95th percentile BRI (1/L) 0.26 0.02 3.10 0.06 1.93 2 2 0.39 0.03 6.65 0.09 3.01 BRI/kg (1/L × kg) 0.008 0.0006 0.15 0.0015 0.06 Discussion 14 15 16 7 8 17 CaOx 7 18 19 2+ 2− 9 10 6 9 20 21 22 23 2 The study has several limitations. So far, the use of our normative data appears to be limited to the Polish population. It is difficult to relate the results to other geographical regions or to different ethnic groups as there are no published data regarding BRI in children. Further comparative and prospective investigations are needed because a cross-sectional study is not able to reveal whether some children with a normal BRI value will be at risk for stone disease in the future. Moreover, some studied children may have had urolithiasis despite normal results of renal ultrasound. However, this study provided consistent normative data on BRI, due to both the large age representation and the stringent selection criteria. In summary, the Bonn Risk Index, expressed as the ratio of ionized calcium to the amount of ammonium oxalate necessary to initiate spontaneous CaOx urinary crystal formation, is lower in healthy children and adolescents aged 3–18 years than in studies of adults by other authors. The BRI during growth appears to be independent of age and sex. Thus, our results may contribute to the effective screening of kidney stone disease in pediatric subjects. We conclude that the BRI may be valuable in the evaluation of pediatric patients at risk for kidney stones, particularly if the BRI from stone formers is demonstrated to be higher than in normal children.