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Nephrolithiasis and renal failure secondary to severe hyperoxaluria were complications of jejunoileal bypass for obesity, leading to the discontinuation of this procedure in the United States in 1980. Bariatric procedures currently in use have not been adequately evaluated for this complication.We compared 24-hour urine chemistry studies of 132 patients with nephrolithiasis who had undergone bariatric surgery with the urine chemistry studies of patients who had undergone jejunoileal bypass, those with routine kidney stones and normal subjects. The primary aim was to determine if hyperoxaluria developed in patients who underwent bariatric surgery and had kidney stones as had been seen with jejunoileal bypass.Patients who have undergone modern bariatric surgery had an adjusted mean urine oxalate excretion of 83 mg per day compared to 39 mg per day for routine kidney stone formers and 34 mg per day for normal subjects (p <0.001 for both comparisons), but not quite as high as that found in patients treated with jejunoileal bypass (102 mg per day, p <0.001). Urine supersaturation of calcium oxalate, the main driving force for calcium oxalate stone formation, was higher in patients treated with bariatric surgery compared to routine kidney stone formers and normal subjects (p <0.001 for both comparisons).Hyperoxaluria is the most significant abnormality of urine chemistry studies in patients with kidney stones who have undergone bariatric surgery. Many of these patients have a degree of hyperoxaluria that could lead to kidney failure. Further studies are required to determine the prevalence of this problem in patients who have undergone bariatric surgery.