Abstract PURPOSE To determine the value of Bonn Risk Index (BRI) in Calcium Oxalate stone formers before and after stone removal. MATERIAL AND METHODS Twenty four hours urinary and stat urine samples were collected from 66 patients with Calcium Oxalate stones before and one month after stone removal with dietary and medical interventions. BRI was estimated in 2 ml of native urine at 37 °C. Ammonium Oxalate 40 mmol/L was added at minute interval with constant mixing in 0.050 ml increments and absorbance measured at 620λ in spectrophotometer. Nucleation was recorded with the increase of absorbance. Ionic Calcium was measured by an Ion selective electrode. BRI was calculated by the formula: BRI = [Ca + 2] mmol/L/ [Ox-2]. BRI > 1/L was taken as risk and BRI of < 1/L as low to no risk for stone formation. Urinary calcium, magnesium, oxalate, citrate, sodium, potassium, phosphate and pH were recorded in all samples. RESULTS The mean BRI before surgery in 24 hour urine was 3.4 ± 1.8/L as compared to post-operative mean value of BRI 1.4 ± 1.4/L (p = 0.002). Mean urinary calcium before surgery was 1.8 ± 0.6 mmol/L vs 1.2 ± 0.4 (p = 0.01) after surgery. Mean oxalate added was 0.8 ± 0.3 vs 1.3 ± 0.5 mmol after surgery. Urinary magnesium showed a hyperbolic correlation with BRI. No correlation was found with citrate. In comparison to 24 hour urine the mean BRI in 14 stat samples was 4.6 ± 5.29 vs 2.1 ± 1.8 in stat urine post-operative. CONCLUSIONS BRI is a valuable tool to assess risk of Calcium Oxalate stone formation and clearly reflects the influence of dietary and medical intervention post operatively. It can be used to monitor effects of dietary and medical interventions.