 Normally the gastrointestinal tract is only sparsely colonized with bacteria. The normal flora of the duodenal or jejunal aspirate contains no more than 105 bacteria per milliliter. Small bowel bacterial overgrowth SBBO is defined as a pathologically increased number of bacteria or the presence of colonic flora in the proximal intestine. The reasons for this condition are manifold, ranging from diabetic neuropathy to surgical bypass. SBBO is frequent in elderly people. Therapy is targeted at correcting the underlying small bowel abnormalities that predispose to the condition and at providing appropriate antibiotic therapy. The symptoms and signs of SBBO can be reversed with this approach. However, in many patients the conditions predisposing to SBBO persist life-long, once present. This suggests that noninvasive, sensitive diagnostic tools with high specificity are required. Bacterial culture of upper intestinal content is considered the diagnostic gold standard. However, since endoscopic harvesting of duodenal or jejunal fluid is difficult and invasive, indirect tests such as breath tests have been advocated as diagnostic tools. Hydrogen breath tests are commonly employed since the substrates can be easily obtained and the measurement is simple. Hydrogen is formed when carbohydrates are fermented in the intestine. Breath hydrogen analysis allows a separation of metabolic activity of the intestinal flora from that of the host, since no known hydrogen production occurs in mammalian tissue. The hydrogen breath test most often used in routine clinical practice uses glucose. However, the utility of this test is mostly limited by its low sensitivity, because there are nonproducers in up to 25 of the subjects tested. The investigators have developed a stable isotope breath test using 13C-labeled lactose-ureide. Glycosyl-ureides are condensation products of reducing sugars and urea in aqueous acid. Lactose-13Cureide has been used to investigate oro-caecal transit time because it resists digestion by small intestinal enzymes and is hydrolyzed by bacterial enzymes in the large intestine. A number of studies have described the use of this substrate in adults and children. The aim of the present study was to investigate the lactose-13Cureide breath test in subjects with suspected SBBO and to compare its results with the results of the glucose hydrogen breath test. Microbiological analyses of upper intestinal bacterial cultures were used as gold standard to identify SBBO. 