To determine whether an elemental diet or prednisolone would be more effective for treating acute Crohn's disease, a controlled trial was conducted on 20 patients with acute Crohn's disease who had never received specific treatment. The first ten patients were put on an elemental diet and the remaining ten were prescribed prednisolone for six weeks at an initial dose of 0.7 mg/kg/day. Patients were assessed using the simple activity index, body weight, erythrocyte sedimentation rate, C-reactive protein and alpha 2 globulin, serum albumin, and radiographic findings of bowel lesions. At six weeks, the patients on the elemental diet showed a significantly greater improvement in the activity index, inflammatory signs such as C-reactive protein and alpha 2 globulin, and radiographic findings of bowel lesions than did those on the steroid. Patients who were given steroids for six weeks and then treated with the elemental diet for four weeks showed improvement in the radiographic findings of bowel lesions and inflammation. The present study strongly suggests that elemental diet is superior to steroids for treating active Crohn's disease.