Purpose: Several studies have documented high incidence of urinary lithiasis after jejunoileal by-pass. Roux-en-y gastric bypass surgery (RYGB) is currently the most common bariatric procedure. Because of its difficult for absorption, RYGB has a potential risk to increase the incidence of lithiasis. This study was conducted in order to test the hypothesis that RYGB increases the incidence urolithiasis after 50% of excessive weight loss. Materials and Methods: We performed a retrospective cohort study to evaluate 58 patients who underwent RYGB at the Obesity Service at Santa Casa de Misericordia de Sao Paulo, between 2000 and 2005, with minimum follow-up of 10 and maximum of 72 months, after the procedure. Results: Forty-five (77.6%) patients had ≥ 50% loss of weight excess. There was no difference between the frequency of urolithiasis before and after the procedure, and nephrolithiasis was observed after surgery in only one patient, however this had been detected before the procedure. Conclusion: In the period studied, RYGB does not seem to affect the incidence of urolithiasis after weight reduction. This may be due to its smaller malabsorptive component as compared with jejunoileal “by-pass”, thereby possibly not significantly influencing the oxalate metabolism.