The irritable bowel syndrome (IBS) is one of the most common conditions seen in clinical practice. The number of effective pharmacologic agents for IBS is limited, and therapeutic innovation is hampered by a lack of complete understanding of the pathophysiology of the syndrome, which is probably heterogeneous. Alterations in the bacterial flora are increasingly considered to be a relevant pathogenetic factor. Consequently, probiotics are being studied as treatment for IBS, but the magnitude of improvement in symptoms with probiotics is limited. Some studies have suggested that there are beneficial effects with poorly absorbed antibiotics, but the results of the studies have been questioned because of issues with patient selection, the choice of end point, and the statistical analysis, and, most of all, because lactulose breath test results were interpreted as indicative of small-intestine bacterial overgrowth.