Background: Obesity alone and rapid weight loss induced by bariatric surgery are recognized risk factors for the development of cholelithiasis. The decision to perform prophylactic cholecystectomy at the time of bariatric operations remains controversial and at the surgeon's discretion. Methods: From June 1998 to April 2001, 103 patients underwent Roux-en-Y gastric bypass (RYGBP) in Hospital das Clinicas/Unicamp (SP). 88 of these 103 patients had their preoperative ultrasonography of gallbladder recovered. 19 of these 88 patients showed gallstones before RYGBP, and the remaining 69 did not have ultrasonographic evidence of cholelithiasis. 36 of these 69 patients were followed with ultrasonography during the 12 postoperative months. They were divided into 2 groups: those who formed gallstones (n=19) and those who did not (n=17), to evaluate the importance of sex, age, preoperative BMI, preoperative excess weight and postoperative percent excess weight loss as risk factors in the gallstone formation. Results: Preoperative incidence of cholelithiasis in the 88 operated patients was 21.6% and postoperative incidence in the 36 patients followed by ultrasonograph was 52.8%. There was no statistical evidence that postoperative gallstone formation is associated significantly with the variables studied. Conclusion: This study confirms the high correlation between morbid obesity, rapid weight loss and gallbladder disease. Predictive risk factors for gallstone formation were not found.