The irritable bowel syndrome (IBS) is characterized by altered bowel habits and abdominal discomfort in the absence of organic disease. No markers exist for IBS, and the definition of IBS is based on the presence of specific symptoms. The Rome II criteria for defining IBS include abdominal pain or discomfort for 12 weeks or longer, which need not be continuous, over the past 12 months plus two of the following: (1) relief of discomfort with defecation; (2) association of discomfort with altered stool frequency; and (3) association of discomfort with altered stool form. Nine percent to 22% of the population report symptoms consistent with IBS. IBS is the most prevalent digestive disease, representing 12% of visits to primary care physicians and 28% of referrals to gastroenterologists.