In a multicenter study of patients with painful functional bowel disorders (FBD), we compared the demographic, health status, and diagnostic features of patients with FBD and developed a functional bowel disorder severity index (FBDSI) for research and clinical care. Two hundred seventy patients with FBD in the United States, England, and Canada were surveyed on symptoms and health status, and their physicians made a diagnosis and rated illness severity as mild, moderate, or severe. Comparisons of 22 demographic and clinical variables were made by study site in addition to physicians' severity ratings. To develop the FBDSI, multiple regression analysis used the demographic and clinical variables to predict the physician's rating of severity. We found that most health status measures of patients with FBD across study sites are comparable and the derived and validated FBDSI scoring system uses three easy to obtain variables: FBDSI = [current pain by visual analog scale (0-100)] + [diagnosis of chronic functional abdominal pain (0 if absent and 106 if present)] + [number of physicians visits over previous six months x 11]. The FBDSI can be used to select patients for research protocols and/or follow their clinical outcome or response to treatments over time.