OBJECTIVE: The importance of psychosocial factors in patients with Functional Bowel Disorders (FBD) has been well-established. However, most psychosocial measures used in research with FBD patients were not designed or validated on this population. A recent international team report recommended that psychosocial measures be developed to increase our understanding and treatment of FBD. The purpose of this study was to develop a reliable and valid instrument designed specifically to assess cognitions of patients with FBD. METHOD: An initial set of 204 scale items was generated from a large pool of thought diaries from patients diagnosed with FBD. Items were additionally refined using several methods, including consultation with a multidisciplinary team of international experts on FBD. The remaining 95 items were administered, along with a set of validating questionnaires, to a new sample of 75 FBD patients in Canada and the United States. RESULTS: The findings indicate that the final 25-item scale has high reliability (Cronbach's alpha = .93; inter-item correlation = .36); high concurrent criterion validity evidenced by the correlation of the scale with a global rating of life interference caused by bowel symptoms (r = .71; p<.001); acceptable convergent validity evidenced by the correlation of the scale with the Dysfunctional Attitudes Scale (r = .38; p<.01); high content validity and face validity; and minimal social desirability contamination (r = .15; NS). CONCLUSIONS: The Cognitive Scale for Functional Bowel Disorders is a valid and reliable scale that can be used as an outcome measure in evaluating the efficacy of different forms of psychotherapeutic intervention for FBD, and can also serve as a helpful assessment tool for health professionals working with patients diagnosed with FBD.