Physicians can usually identify irritable bowel syndrome (IBS) from patients’ symptoms. Many patients additionally require only routine blood tests and a colon evaluation, and some require no tests at all to secure the diagnosis. However, some patients worry that they could have another cause for their symptoms, especially when symptoms are severe and chronic, or they know other people who they think had similar symptoms but a different disorder. Occasionally, another medical problem mimics IBS symptoms. This discussion focuses on how IBS is diagnosed and distinguished from other disorders. Reviewed and updated 2009.