- OBJECTIVE: The purpose of this study was to evaluate the utility of the Functional Bowel Disorder Severity Index (FBDSI) as a measure of severity of disease among patients with the irritable bowel syndrome (IBS) and matched controls. METHODS: A total of 75 IBS patients and 69 matched controls completed questionnaires on bowel symptoms, health status, quality of life, psychological distress, concerns, anxiety, and sense of coherence. All participants also were tested for fibromyalgia (FS), a functional disorder of the musculoskeletal system. All participants were administered a questionnaire that included the FBDSI. On the basis of their responses to the questionnaire, the controls were subdivided as healthy controls (n = 48) or IBS nonpatients (n = 21). On the basis of the FS classification, 75 IBS patients were subdivided as IBS only (n = 50) or IBS and FS combined (n = 25). RESULTS: The mean FBDSI score was higher for the IBS patients than the controls (100.5 ± 12.7 and 23.5 ± 3.9, respectively; p < 0.001). IBS nonpatients had an intermediate score of 42.3 ± 18.0. Patients with both IBS and fibromyalgia had the highest mean FBDSI score: 138.8 ± 31.5. There was no association between FBDSI and age or gender, but FBDSI was significantly associated with other measures of health status. CONCLUSIONS: An association was found between the FBDSI and IBS patient status: IBS nonpatients, patients with IBS only, and patients with both IBS and fibromyalgia had increasingly severe scores. The results provide support for the validity of FBDSI as a measure of illness severity in functional gastrointestinal disorders.