Needing a single index of degree of illness in Crohn's disease, the National CooperativeCrohn's Disease Study group collected data prospectively from 187 visits of 112 patients with Crohn's disease of the small bowel, colon, or both. Information on 18 predictor variables was gathered at each visit. In addition, the attending physician rated his over-all evaluation of how well the patient was doing and compared the patient's status with that at the previous visit. A multiple regression computer program was utilized to derive an equation for prediction of the physician's over-all ratings from a subset of the predictor variables fulfilling a combination of constraints. This equation, numerically simplified and utilizing eight selected variables, is the Crohn's Disease Activity Index. Index values of 150 and below are associated with quiescent disease; values above that indicate active disease, and values above 450 are seen with extremely severe disease.