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Abstract: The human gastrointestinal microflora is a complex ecosystem with about 500 different bacterial species. In healthy individuals, the human stomach and the proximal small bowel contain only a few bacterial species, with the terminal ileum considered a transitional zone between the proximal small bowel aerobic microflora and the colonic anaerobic bacteria. The colon hosts a complex and variegate microbiota, including anaerobes (bacteroides, bifidobacteria, lactobacilli and clostridium), and several other species. The enteric microflora is involved in protective, trophic and metabolic functions. The interaction between gut microflora and substrate leads to gas production, while their overproduction can be responsible of the “gas-related syndrome”, a constellation of non-specific gastrointestinal symptoms (bloating, borborygms, flatulence, abdominal distension and discomfort). Any condition leading to the perturbation of the equilibrium between enteric flora and the surrounding system is a predisposing factor for bacterial overgrowth. Proposed antimicrobic treatments, including tetracycline and norfloxacin, amoxicillin-clavulanic acid and S. Boulardii, are still highly empiric. The efficacy of rifaximin, a non-absorbable antibiotic with bactericidal action against anaerobes and aerobes and a low toxicity, has been evaluated in patients with small intestinal bacterial overgrowth and gas-related syndrome, and has a potential therapeutic role in a subgroup of patients. [Copyright &y& Elsevier]