This work focuses on the in vitro calcium-oxalate (CaOx) crystallization behaviour of native and synthetic urine samples in order to establish a CaOx crystallization risk index for unprepared native urine. Native 24-h urine samples from healthy persons and from stone-formers were examined. Within a [Ca2+] versus added oxalate (Ox2-) diagram, we observed fields which allow the discrimination of each urine sample in terms of more or less risk. The [Ca2+]/(Ox2-) ratio is calculated and termed the "Bonn-Risk Index" (BRI; per litre). We propose that BRIs > 1/l denote samples "at risk", whereas BRIs < or = 1/l denote those "without risk". Second. the effects of different concentrations of citrate and Mg2+ on BRI were investigated in artificial urine. The transferability of BRI between native and synthetic urine samples is proved. To evaluate the impact of the proposed BRI, it is compared with the more familiar relative urine saturation index calculated for CaOx and brushite. Urine sampled from stone-formers shows risk indexes between 0.278 and 23.0/l (mean 2.87/l), while urine from healthy persons varied between 0.060 and 4.890/l (mean 1.05/l). Comparing the results of healthy volunteers and patients, the significance of BRI and relative urine supersaturation (RS) with respect to CaOx is P < 0.0005 and P = 0.013, respectively. Fast and easy to perform, determination of the risk index is a suitable tool for estimating the actual CaOx formation "status"--"at risk" or "without risk"--from the native urine of any person.