518 Pediatr Nephrol (2007) 22:514–520 Fig. 3 Bonn Risk Index (BRI) in the whole studied group (left box) and in separate 1-year age groups for children aged 3–18 years between the values of BRI/1.73 m2 and BRI/kg in the various age groups or in relation to sex. Discussion Urinary stones are considered a major health problem in society, both in adults and in children, due to their recurrent nature and the cost of treatment [14]. The pathogenic pathways leading to stone formation in kidneys have not Table 1 Bonn Risk Index values in children aged 3–18 years in relation to body surface area and body weight BRI (1/L) BRI/1.73 m2 (1/L×1.73 m2) BRI/kg (1/L×kg) Median Minimum Maximum 5th 95th percentile percentile 0.26 0.39 0.02 0.03 0.008 0.0006 3.10 6.65 0.15 0.06 0.09 0.0015 1.93 3.01 0.06 been fully explained. During processes of calculus forma- tion, a number of phenomena have been reported such as excess of crystallizing substances, nucleation, crystalliza- tion, aggregation and stone formation [15, 16]. However, it has not been explained why calculi do not form in all subjects, despite a large amount of urinary crystallization products. Over the past few years, there have been many attempts to define the risk factors leading to urinary stone formation on the basis of the ability to form oxalate crystals, a main component of calculi [7, 8, 17]. Tiselius et al. described the practical importance of assessing the activity of calcium oxalate ions in urine using the APCaOx index [7]. Stone formation has also been evaluated in terms of excess urinary calcium oxalate, using the computer pro- gram EQUIL [18, 19]. Laube et al. showed a strong correlation between the concentration of free calcium ions [Ca2+] in urine and the quantity of ammonium oxalate (Ox2−) added to invoke spontaneous urinary crystallization of CaOx [9]. The authors suggested that the BRI index in healthy subjects was significantly lower than in those forming urinary calculi. The study, based on a group of 72 healthy subjects, provided mean BRI values of 1.05 ±