This article reviews the evidence for validity of symptom-based criteria (Manning, Rome I, Rome II, and Rome III) for irritable bowel syndrome (IBS). Two kinds of validations are reported: (i) studies testing whether symptom criteria discriminate patients with structural disease at colonoscopy from patients without structural disease; and (ii) studies testing whether symptom criteria discriminate patients presumed to have IBS by positive diagnosis from healthy subjects or patients with other functional and structural disorders. The first study type addresses an important clinical management question but cannot provide meaningful information on the sensitivity or positive predictive value because IBS is defined only by exclusion of structural disease. Specificity is modest (about 0.7) but can be improved to 0.9 by the addition of red flag signs and symptoms. The second type of study judges validity by whether the symptom criteria consistently perform as predicted by theory. Here, factor analysis confirms consistent clusters of symptoms corresponding to IBS; symptom-based criteria agree reasonably well (sensitivity, 0.4-0.9) with clinical diagnoses made by experienced clinicians; and patients with a clinical diagnosis of IBS who fulfill Rome II criteria have greater symptom severity and poorer quality of life than patients with a clinical diagnosis of IBS who do not fulfill Rome criteria. There are no consistent differences in sensitivity or specificity between Manning, Rome I, and Rome II. Both study types support the validity of symptom-based IBS criteria. Tests of Rome III are needed.