Irritable bowel syndrome (IBS) is a chronic disorder with symptoms that range in severity from mild and intermittent to severe and continuous. Although severity is a guiding factor in clinical decision making related to diagnosis and treatment, current guidelines related to IBS do not address the issue of severity. Recent data suggest that severity as a multidimensional concept, not fully explained by intensity of symptoms, has important clinical implications including health care utilization and health-related quality of life. Components of IBS severity include symptom intensity, time of assessment, whether the patient or physician makes the severity determination, the type of scale used to measure severity, and the degree of disability or impairment. Currently no consensus definition of IBS severity exists, although 2 validated scales of IBS severity have recently been published. Review of the literature suggests that the prevalence of severe or very severe IBS is higher than previously estimated with a range from 3%-69%. Individual IBS symptoms are important but are not sufficient to explain severity. Rather, severity has multiple components including health-related quality of life, psychosocial factors, health care utilization behaviors, and burden of illness. However, studies have not been adequately designed to determine the relative values of these factors in IBS severity.