We have recently shown that the majority of patients with irritable bowel syndrome (IBS) have an abnormal lactulose breath test to suggest the presence of bacterial overgrowth of the small intestine. In open label and double blind treatment of IBS subjects with antibiotics, a dramatic improvement in clinical symptoms are observed. In these studies, the antibiotic chosen was neomycin, which is noted to have an efficacy of 20-25% in normalizing the lactulose breath test. A more efficacious antibiotic is needed. Therefore the aim of this study is to determine the efficacy of rifaximin in normalizing the lactulose breath test in IBS subjects with concomitant improvement in clinical symptoms.