Irritable bowel syndrome (IBS) has not been associated with any pathopneumonic structural, biochemical, or serologic abnormalities. In addition, the symptoms associated with IBS mimic those of many other diseases. Physicians, therefore, face a challenge when presented with a patient who has symptoms consistent with IBS. The physician must be able to correctly identify patients who truly have the disorder, while excluding patients for whom symptoms are related to a non-IBS cause. Standard diagnostic criteria, such as the Manning and Rome criteria, greatly facilitate the appropriate diagnosis and treatment of IBS.