In evaluating the medical literature dealing with antibiotics and inflammatory bowel diseases, I cannot help but recall the adage: "Those who have enthusiasm have no controls and those who have controls have no enthusiasm." There just are not enough data to justify the use of most antibiotics in the treatment of most patients with Crohn's disease or ulcerative colitis. An increasing body of data does support the use of metronidazole in selected patients with Crohn's disease, especially those with perianal disease or fistulae. However, this drug has important side effects that may preclude its long-term use. Other antibiotics have been inadequately tested and there is not adequate evidence to support their use.