CD is thought to arise from a combination of inherited (genetic) factors and some undefined environmental factor(s). One environmental factor that has been shown to be intimately involved with the development of CD is the presence of bacteria that normally inhabit the intestines. As a result, some physicians have tried to alter the normal bacterial population as a means of controlling the inflammation (swelling) in the intestines of individuals with CD. Among such strategies is the use of a combination of metronidazole and ciprofloxacin. These broad-spectrum antibiotics control CD symptoms by acting on the intestinal bacteria that can contribute to chronic inflammation. More investigation is needed to firmly establish the usefulness of this therapy because previous clinical trials have given mixed results, although they have suggested that antibiotics can be particularly useful in cases of Crohn's colitis (CD that primarily affects the large intestine). Because these earlier studies have lacked a large enough patient population with colonic involvement, a trial focusing on this CD subgroup with a sufficient number of subjects will help to clarify the value of combining metronidazole and ciprofloxacin.