An objective and quantitative standard of inflammatory activity for patients with Crohn's disease was developed. This Activity Index (AI) was derived from data of 63 patients with Crohn's disease who had been submitted to a total of 85 clinical examinations. On the basis of 18 predictor variables three physicians gave an overall evaluation of the severity of inflammatory activity in each patient. Stepwise multiple regression analysis was used to investigate which combination of variables contributed most to the overall evaluation. The combination of the following nine variables gave a very good correlation (r = 0.95) with the overall evaluation: serum albumin, ESR, body weight related to length, abdominal mass, sex, temperature, stool consistency, bowel resection, and extraintestinal symptoms related to Crohn's disease. This combination of variables expressed in a score that was used as an activity index proved to be very useful in the assessment of disease activity and of the effect of therapy. Index values below 100 are associated with inactive disease, values between 100 and 150 can be regarded as indicating slight inflammatory activity, values between 150 and 210 as indicating moderate, and values above 210 as indicating severe-to-very-severe inflammatory activity.