51 patients with Crohn's disease who were in good health while taking azathiprine, 2 mg/kg body-weight/day, for at least six months were allocated either to a group in which azathioprine was continued or to one in which a control tablet was substituted. The trial lasted one year unless relapse recurred earlier. The cumulative probability of relapse was nil at six months and 5% (+/-5 S.D.) at a year among those on azathioprine, compared with 25% (+/-9 S.D.) at six months and 41% (+/-11 S.D.) at a year among those in the control group (P less than 0.01). 1 patient in whom azathioprine was continued died of pancytopenia in the fourth month of the trial. Azathioprine is potentially toxic but appears to reduce the relapse-rate in Crohn's disease.