The effect of azathioprine in patients with active Crohn's disease has been evaluated in a double-blind cross-over trial. Fifteen patients were treated with both azathioprine and placebo: only two improved while on azathioprine, six got worse, and in seven there was no change. The drug seemed to be the cause of deterioration in two of the patients. Although the drug may be of some value in a few patients, it should be used with caution and only when other measures have failed.