Although antibiotics are clearly recognized as having a role in treating the infectious complications of inflammatory bowel diseases (IBD), their impact in the primary treatment of IBD has long been an area of speculation. Over the past decade there is increasing evidence that luminal gut bacteria play a role in the pathogenesis of IBD, particularly Crohn's disease. Compelling evidence that normal commensal bacteria induce chronic intestinal inflammation in susceptible rodents provides an excellent rationale for treatment of human IBD with antibiotics. This article summarizes published studies of antibiotics in IBD patients and reviews available data for the use of antibiotic therapy in Crohn's disease and ulcerative colitis.