A comprehensive assessment including subjective and objective parameters seems to be essential for evaluation of disease activity in Crohn's disease. The aim of the present study was to develop an activity index (AI) of Crohn's disease, composed of both subjective and objective variables. Data composed of a total 132 clinical examinations in 100 patients with Crohn's disease were used to determine the AI. Three physicians independently evaluated disease severity in each patient. Stepwise multiple regression analysis was carried out with the overall evaluation as a dependent variable, and with 18 parameters as independent variables. Analysis showed that the following seven variables had a significant correlation with physician's overall evaluation: abdominal pain, abdominal tenderness, complications, C-reactive protein (CRP), body temperature, diarrhea, and body mass index (BMI). AI was expressed as follows: AI = 3.5 x abdominal pain + 3 x abdominal tenderness + 3 x complications + CRP + 3 x body temperature + 4 x diarrhea - 0.4 x BMI. AI values below 15 corresponded to inactive disease, values between 15 and 25 to mild disease, values between 25 and 35 to moderate disease, and values above 35 to severe disease. This study suggests that the activity index is useful for evaluation of the effect of medical treatment in Crohn's disease.