To determine the efficacy of azathioprine in the treatment of Crohn's disease, a 26-week double-blind trial was performed. 20 patients with Crohn's disease, requiring at least 10 mg of prednisone/day over the 3 months prior to entering the study were randomized into placebo (10 patients) and major criterion of success in the trial. There were 7 relapses in the placebo group (5 patients) and 2 relapses in the azathioprine group (2 patients). Complications including fistulae were not affected by the medications. The mean reduction in steriod dosage in the azathioprine group at the end of the trial (-15.5 mg) was greater than in the placebo group (-6.1 mg). These results suggest that azathioprine may permit reduction or discontinuation of steroids without the worsening of symptoms in some patients who appear to require steroids for control of their symptoms. The clinical features of this "AZA-responsive subgroup" remain to be defined.