The duodenal and fecal flora were studied in 18 patients suffering from gastrointestinal disorders during treatment with an elemental diet (Vivasorb). Before treatment the duodenal flora was found to be abnormal in 6 of 17 patients. After treatment the flora remained abnormal in 2, whereas, it became normal in 4 patients, 3 of whom were suffering from ulcerative colitis. The fecal flora was uninfluenced by the diet. The overall number of colonies per gram of dry feces did not vary significantly, whether anaerobic or aerobic. The ratio of anaerobes to aerobes was significantly changed in only 3 patients. There was no correlation between changes in the duodenal and the fecal flora, on the other hand, and the final clinical result of treatment with elemental diet on the other hand. On an average, daily fecal output remained unchanged. In several cases the volume either increased or decreased, but the fecal flora was unchanged. The findings are in agreement with those made in most of the earlier studies of healthy volunteers and in one study of a patient who was suffering from ulcerative colitis.